I wouldn't go as far as saying retarded, because you have to realize that some people (like me) thought their first album was little more than a rip-off of the Jeff Beck Group. Of course, they got much better over the years, but there are folks that will still hold that (along with the group's smugness and general lack of concern for their audience, two traits that Van Halen has unfortunately inherited) against them.
It's inevitable that blues purists would bristle at the band's heavy-handed takes on the medium, which certainly lacked a great deal of subtlety at times. The thing is, Zep knew when to hold back. Look at Beck, Bogert & Appice. The clods in that rhythm section had zero clue as to when to lay off, and their version of "Superstition" (along with their various blues numbers live) proves it. Rather than attempting a cool funk groove, they positively flattened the damned song. Yeah, there is a place for bombast, but not on every song in your catalogue.
So people might tend to think of Zep as an overblown group, as all they hear is the heaviness. And Plant and company could be a bit acrobatic and overwrought occasionally. That coupled with the highly questionable personal habits of most of it's members, and it's relatively easy to see why some people tend to dismiss the band, if not outright loathe them.