None of their songs appear in Rolling Stones top 200 (I didn't check back further than that), none of their albums are in the Rolling Stone top 100, and they're not listed in the Top 100 artists. So certainly they're certainly not on "any" top 100 lists for best bands and best songs. I'm not saying that Rolling Stone is the gatekeeper for what a great song, album, or band is, but it's your metric.
To use
@Randolphkeys 's standard, I'd agree they made "quality rock songs". But so did .38 Special. I just don't see their songs as being significantly different/better than those made by a shitload of other hair bands. Is "Pour Some Sugar on Me" really any different/better than Whitesnake's "Here I Go Again", Poison's "Every Rose Has Its Thorn", or Bon Jovi's "You Give Love a Bad Name"? All of those songs could have been written/performed by any of those bands. There's just a sameness. Saw one of their shows once back when they were big, and it was...fine. But nothing special.
The big separation for Def Leppard over bands like Whitesnake and .38 Special would be sheer album sales. I'd like to add that "Here I Go Again" is a great stand alone number, but that is a topic for another day. RIAA has Def Leppard at 35.5 million albums sold. Queen and Bon Jovi are at 34.5 million sold and are both in the HOF, for comparison. Obviously Poison and Whitesnake never had that type of selling power. Of course we shouldn't just go by album sales or else we would have to talk about Celine Dion and Barbara Streisand as a R&R HOF candidate.
With all these musicians, throwing around statistics or sales numbers turns into an excercise in ignoring facts and pushing other facts forward. By talking album sales numbers, I conveniently left out that Foreigner outsold Def Leppard by two million albums. Why? I dislike Foreigner. My wife defends Foreigner in discussions of music when the subject comes up and thuroughly embarrasses me when she does it.
The fact of the matter is that all these bands up for enshrinement have been passed over for the R&R HOF at least once. They all have a legitimate argument against their inclusion. For Def Leppard, they were at the Poison type level until they brought in Phil Collens (not Collins) on guitar for Hysteria as their original main guitarist Steve Clark became a drugged out waste. Then as grunge and alternative influences hit, they didn't really change their sound. They peaked and faded, as have plenty of other bands.
Well, no argument there but that was kind of the point. I think what the standard has become over the years blows. I dunno -- I just think that if I'm listening to a Hall of Fame artist, they should be doing something that sounds unique/different/of exceptionally high quality, etc.. Not something that pretty much sounds indistinguishable from what a dozen other bands did.
Def Leppard had a fantastic producer who put layers of guitar effects over the songs. When you said you saw them live and it was underwhelming, I wasn't that surprised. In videos I've seen of them live they rely heavily on the crowd carrying them through. It does speak to how much the music connected with their fan base, but overproduction was a trait of mid 80s through early 90s albums.
I usually defend bands that had influence and originality over the pure hits as well. If we were talking The Pixies or Def Leppard, only one would be allowed in, I'd cast my vote for The Pixies who charted the course of alternative pop songs in a new direction. It's also why I liked the inclusion of DEVO - they pioneered experimenting with the genre through the late 70s into the mid 80s, even if some of it didn't work. They were known for being discovered by David Bowie and jamming with plenty of rock giants like Neil Young who were stuck in their ways a little too much and helping them rethink their sound. DEVO didn't have many hits, but they helped others make hits.
It seems like the Rock Hall has different types of acts getting in every year to address different criteria. They let in a hit machine like Green Day and an influential individual in Lou Reed the same year, but also let in Bill Withers, Stevie Ray Vaughn and Joan Jett because they wanted to appeal to a variety of musical tastes. The HOF overall caters to somebody like me who listens to white guys noodling away on electric guitars, but they want other styles recognized as music genres become less fragmented.