Collen may say that, though I'd be curious to get a quote on his exact words. Elliott, Savage, and Lange tell a different story. And it's not an opinion, man. Go and look at the songwriting credits for their classic albums. It's common knowledge that Lange had a HUGE influence on every aspect of the band, particularly from Pyromania on. Read the band talking about the making of Hysteria. You'll see just how much of a collaborative effort it was. Willis also had a big influence on the band's sound in the early days. He was a great songwriter and rhythm player in his own right. Most of the backing tracks on Pyromania were his.
Not taking anything away from Clark; he was an important part of their sound. But he wasn't the only part. Vivian Campbell has described him as a great "bits" guy -- that is, he didn't usually bring in complete songs (the odd exception like "Wasted" aside), but he brought in killer riffs and song pieces that the band could use when composing songs.
As for Def Leppard "not doing much" since Hysteria, uh, they've actually produced more albums since then. Obviously they don't sell like they did in the old days, for a variety of reasons, one of them being the death of hard and glam rock in the '90s, the other being the collapse of the traditional music industry and the rise of Spotify, etc. But they have continued to produce. In fact, the 2015 self-titled album is really good. But if you think they turned into a "teen pop nipple glam band," you probably aren't open enough to check them out. For me, Hysteria is the peak of their sound. The song strength, the production, the wonderful interplay between Clark and Collen -- all those inversions and harmonies and supremely memorable solos -- it stands the test of time. I like On Through the Night and High and Dry for what they are, but the band would never have made their mark if they'd stayed in the safe realm of pure hard rock. On Pyro, Hysteria, and Adrenalize, they essentially cemented themselves as Queen meets AC/DC (that's Elliott's description), and it's phenomenal.
They're also still amazing live, and truth be told, part of that is down to Vivian Campbell replacing Clark. Campbell, besides being a superb guitar player, is a stronger backup singer than Clark ever was. So it's a lot more complicated than "Clark was a guitar hero, they'd have been nothing without him." I get the appeal of that narrative; it's just not true, in this instance.
