Well, the statements are phrased as if they were mutually exclusive, when in fact both are true.
Dylan wrote some great stuff, he plagiarized some pretty good stuff too (actually, rather a lot of stuff, and he's never been seriously called on it, though it's not exactly a secret)... and he wrote some mediocre stuff, and some utter crap.
As far as his singing... Well, sometimes it was the right voice for the song (Subterranean homesick blues, rolling stone), sometimes it wasn't a particularly good voice for the song (Tangled up in blue, Maggies Farm), and sometimes his voice is bloody awful for the songs (Watchtower, Masters of War).
On "Watchtower" Dylan himself said (and I'm paraphrasing since the quote is reported differently from different sources) something like "I wrote this song for Jimi Hendrix. I didn't know I was writing it for him, but now it's clear".
I think his lasting legacy once the baby boomers are gone, will be Highway 61 Revisited, Blonde on Blonde, a few singles from other albums and all the songs he wrote and then other people did something with (dozens).
That said... You have to remember the times a lot of these songs were released.
Dylans first real hits came out in 1963. "A Hard Rains Gonna Fall", "Blowin in the Wind", and "Girl from the north country".
For these songs to break through into the main stream (mostly on the success of other people singing his songs actually) in 1963, was incredible. The top songs of 1963 were "Sugar Shack" by the Fireballs, and "Surfin USA" by the beach boys (Dylan himself didn't make the top 100 singles, but Peter Paul and Mary did singing Dylans "Blowin in the Wind", and the album hit number 22). Other popular selections from that year included Sukiyaki and Andy Williams "cant get used to losing you".
"Can't get used to losing you" vs. "Blowin in the wind"...
We think of the '60s as a time of great social change; but if you look at pop culture, most of this change didn't really burst onto the public consciousness until 1967 or 1968... and has been point out by many, most of what the public thinks of as "the sixties" didn't really happen in most of the 60s... it was mostly from '68 to '74.
But Dylan was laying it down in '63.
In '64 he released "the times, they are a changin", and "It Aint me Babe"... Top 3 songs of '64: "I want to hold your hand" and "She Loves You" by the Beatles, and Louis Armstrong doing "Hello Dolly".
'65... Dylan releases "Subterranean Homesick Blues" off of "Bringing it all back home", and then the album that breaks him into the wide world "Highway 61 revisted" followed 8 months later by "Blonde on Blonde".
Top song of '65? Wooly Booly by Sam the Sham & the Pharoahs. "Like a Rolling Stone" was number 41. '66? Barry Sadlers "Ballad of the Green Beret". Dylan hit 82 with "Rainy Day Woman".
That was the peak of his career. It was almost two years before he would release another album (December of '67 with "John Wesley Harding"), and by then the world had caught up to Dylan, and he wasn't the revolutionary any more. He didn't make the top 100 that year... though "The Monkees" did, four times...
In fact his next top 100 single wouldn't be 'til '69 with "Lay Lady Lay" off "Nashville Skyline" (a polarizing album, that I actually like, but most critics and casual fans didn't... it still made it to #3 on the album charts), and it would be his last top 100 charting single (though his albums continue to do well even today).
Which just reinforces that what charts isn't necessarily what people think of later as "great music"...
... though actually, as an aside, I need to defend the Monkees.
First, I really like a lot of the Monkees stuff. It was great songwriting, from professional songwriters like Goffin and King, Lieber and Stoller, Boyce and Hart, Neil Sedaka, Neil Diamond etc... And at the very least competent, and even sometimes excellent musical performances. Peter Tork and Mike Nesmith were both professional musicians (both played guitar and sang, Tork also played Bass, Banjo, Violin, Piano, and Organ). Micky Dolenz was a child actor, but after lessons and some time with studio musicians was actually a decent drummer. Davy Jones, also a child actor, played piano, drums and bass. Both Dolenz and and Davy Jones could sing quite well; both having distinctive voices that worked very well for their particular songs.
The DID in fact play their own instruments, though they ALSO used session musicians. They were a real band, and are derided for not playing or writing their own music etc... Well, big secret folks, that's how almost every band from a major label worked in the 50s and 60s. That's what A&R people did. They took artists, and combined them with repertoire from the record companies catalog or contract songwriters. Most artists never wrote their own music... and in fact even today most pop artists still don't... and on studio records of the time, most artists used session players for various parts, and again they still do.
The Monkees actually had to threaten to walk out on their contracts if they weren't allowed to put their own music, and play their own instruments on the records, from the second album on (and even on the first album, they used Guitar from Tork and Nesmith). Although not every track had every part by a Monkee, at least half the tracks on all the remaining albums featured instrumentals from Tork and Nesmith, and by the last four albums they were doing mostly original composition.
People often called them some variant of "fake beatles", but in fact they were good friends with the Beatles; and all of the Beatles have (or had) repeatedly said in interviews how much they liked and respected the Monkees both as guys, but also musically.At any rate, Dylan never really had much single success, but his albums have always charted well. He only ever missed the top 100 albums in '62 with his first release (which didn't chart, even when re-released after his later top ten successes); and in 34 studio albums has only missed the top 40 albums 4 other times (the widely hated "Knocked out loaded" and "down in the groove" from the early 80s, "good as I been to you" and "world gone wrong" both in the 90s) and other than his first uncharting album, never missed the top 100 ...with even his awful Christmas album making number 23.
The Monkeys put out 9 albums in 4 years of generally quite good pop rock music, all of which were in the top 40 albums, and the first four of which hit number one on the billboard album charts. Yes, they did it with a strong studio backing them, and a TV show, and outside songwriters etc... But that is no less difficult than many other pop stars have had to do, and any way you take it, is an achievement worthy of some respect.
Oh and take this as you will... But Mike Nesmith has gone on to a very successful career in music both as an artist and a producer. He created the concept of "music videos" as we know them today, made a successful venture out of it, and then turned it over to Warner cable, who turned it into... Yes that's right... Mike Nesmith came up with MTV, and was the producer, co producer, or executive producer on a couple hundred of the early videos played on the network. He also won the first Grammy given for a video.
When albums chart well and singles don't that usually means you aren't getting a lot of mainstream airplay; and your albums are mostly being purchased by devoted fans. That seems to have always been the case with Dylan, outside of the period 1964-1969... and really '65-68, which can be seen as his "mainstream rock" period.
But man.. from '63 to '66... it really was revolutionary what he was doing. THAT is what makes him legendary.