About the best way I'd put it is that it's an inappropriate style for superhero comics, which tend to favor somewhat-exaggerated-but-still-more-or-less-within-normal-bounds-proportioned figures and clean lines. There's a lot of wiggle room within that visual genre--the two most popular artists in superhero comics in the eighties were probably John Byrne and Frank Miller, who had very different approaches to it--and comics on the satirical end of superheroics can often get away with a more "cartoonish" approach. (I'm thinking especially of the artists that are working with Charles Soule on Thunderbolts and She-Hulk.)
You know who Liefeld reminds me of a lot? Don Simpson, who's probably best known within comics for his eighties series Megaton Man (eventually, the character would be published by Image Comics, which Liefeld helped create) and the one-shot story with Alan Moore, "In Pictopia", which decried the grim 'n' gritty era of comics which Moore helped create (Moore, of course, would eventually end up working for Liefeld, revising some of Liefeld's creations--most notably Supreme, a thinly-disguised Superman ripoff--into less grim-n-gritty characters). The thing is, Liefeld seemed to think that the muscles-on-his-muscles approach that Simpson used in an obviously satirical fashion was actually pretty cool.
Still, though, that sort of art could have passed muster in a section of the comics industry that wasn't devoted to the superhero aesthetic: undergrounds and independents. Weirdo, for instance, regularly featured artists who were way worse than Rob, and "oddly-proportioned characters with an excessive amount of superfluous detail" is another way of describing much of Robert Crumb's output in the eighties. (You want a female character who's exaggerated in almost every way? Meet Mode O'Day.) But, of course, that would have paid a lot less, and I don't think that Peter Bagge or Dori Seda were ever offered blue jeans commercials.