Does Liefeld's popularity in the 80s and 90s show how bad comic art was in general? In some ways I think comic art is so much better these days, with more depth, colour and detail.
Liefeld couldn't have gotten a job in comics until the '90s. He'd have been laughed right out of the bullpen.
Comic art was always competent from the 60s through the 80s. It also had a consistent "house" style - while artists' work was identifiable, it always fell within a rather realistic style. Steranko laid the seeds for that to change, and Miller and a few others made headway, but it was the early 90s when the advent of new print technologies, the influence of other media and the push to be as prolific as possible all combined to create the explosion of styles and artistic media that we see in comics now - and with it, the full gamut of quality, from absolute shit to truly mind-blowing artwork.