It depends on what you mean by "well-regarded" and by whom. Liefeld was part of a new vanguard of comic artists whose new approach to art and plotting defined the 1990s style of comics. This new style of art was characterized by:

• Splash pages. Art that had previously been done in a series of small panels was transformed to large spreads that often covered all or parts of two adjacent pages.
• Exaggerated anatomy. On male characters, this was muscles on top of muscles, and on female characters, enormous breasts and tiny waists. Liefeld in particular was known for these exaggerations. His Captain America is a quick lesson in 90s comic art by itself.
• Large guns, even on characters who didn't normally use them

The new style was accompanied by storytelling that was often darker and grittier than previous. (Note that at Marvel Comics in particular, the artist had a lot of responsibility for storytelling, with the writer expected to do dialogue that went with the art, not vice-versa).

These changes did not sit well with everyone, but they unquestionably were associated with the largest sales numbers the industry had ever seen, and would ever see.

A great case study in both 90s comics in general and Liefeld in articular comes from the Marvel book The New Mutants. New Mutants was a spinoff from the X-Men that had been around since 1982. For its first 97 issues it had been written by either longtime X-Men writer Chris Claremont or his close associate Louise Simonson. By the late 80s, the book was falling in popularity compared to the rest of the X-Men line. The October 1989 sales figures show the book had slipped to the low teens, well behind similar titles. Not long after this, Liefeld was brought in as penciler and sales began to increase. The partnership between Simonson and Liefeld did not go well, and Simonson became distressed when the art she received from Liefeld did not match the plots she had sent him. Simonson left the book, which was turned over to Liefled, and rebooted as X-Force.

X-Force went on to become a hit for Marvel. Five of the top twenty selling books for the entire industry in 1991 were X-Force titles plotted and drawn by Liefeld, and X-Force #1 remains the second best-selling book of all time. Money talks, and the remainder of the 90s and early 00s was driven in a large part by the art aesthetic that Liefeld helped to drive. Liefeld, along with Jim Lee and Whilce Portacio, eventually formed Image Publishing, which allowed comics creators to retain ownership of their own creations. Their success as creators helped redefine the relationship between traditional comics publishers and writers and artists.

Not everyone was enamored of Liefeld's art. Lists like this one were circulating even on the primitive Internet of the time. And eventually, the 90s aesthetic and its excesses went away too. Without the massive sales numbers, Liefeld and his art is viewed with a more critical eye, and things like the exaggerated muscles, wasp-waisted women, and inability to draw feet has either raised ire or a humorous reaction today.

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Some material drawn from "Chris Claremont's X-Men", 2018, Respect Films