Relatively speaking, the Ramones' first LP sounded much more pop-oriented compared to later albums, but you could say that about many artists.. but if you compare it to let's say any of the Billboard Top 100 of 1976, it's easy to see that the opinion of Ramones early music is based upon the bias of their music history and evolution in a vacuum -- But Pop music in '76 was really not much like this music, if you look at the list.
Other tunes on the same album (ex: "Now I wanna Sniff Some Glue) were pretty much run of the mill Punk Rock (and I don't mean that in any negative light).. I think if anything, perhaps their early music might be viewed in this light is simply because of the upbeat sound of one song -- Blitzkrieg Bop -- And the catchphrase "Hey Ho, Let's Go!". It is timeless, and has become one of the most iconic songs in Ramones history (although ironically, it didn't chart at all).. That said, the song in my view isn't really representative of any bigger picture, it's just a song with a catchy hook that America latched onto, and it became this thing. "I Wanna Be Your Boyfriend" is demonstrative of how they enjoyed sixties rock, and they have stated this influence on many occasion (Stern had them on regularly, this comes to mind).. This was 1976, and essentially the beginning days of Punk Rock in America, so the evolution of the Ramones was in a way the evolution of Punk Rock, and just as Blues and Doo-Wop evolved into Rock and Roll of the 1950s, but within ten years you had bands like the Beatles doing complex Rock like Sgt. Pepper. That's a lot of change in ten years -- While it feels fast now, back then it all felt natural for us fans, other than the standard "I hope this album is good" sort of thing.
In those ten years, Ramones released nine albums (eight in the studio), so easier even yet to see how a sound will change, as nine albums in ten years is staying pretty busy.
Since the Ramones first album largely marks the beginning of the PR era in America, you can see from that time in the next ten years it has gone from simple three chord tunes to being more politically astute (Bonzo Goes To Bitburg), tighter sounding and all that. Again, natural evolution, since any genre becomes more refined (and perhaps defined) in time.. But they never stopped doing covers of the music they loved, and that was sixties stuff.
I saw them maybe thirty years ago, and I always thought of them -- Even if I hadn't heard of all those interviews, as guys who loved the simple and sweet rock of the sixties (before and after), and they took that sound and infused it with what was already going on (pretty much like other music as well).
I always enjoyed them so much because as well as the music, Joey Ramone just seemed to be the most gentle of souls, which if you think about it, seems antithetical to the premise, but who are we to say? Since these guys had a hand in writing the book.
Not a musical historian or anything, just an audiophile. I hope I helped.