Frank Brunner was an artist who really blew my socks off. Not being much of a Marvel reader, I didn't have many opportunities to find his work, but I happened to get a second-hand issue of MARVEL PREMIERE with his Doctor Strange and it was stunning. I bought HOWARD THE DUCK mainly on the strength of Brunner's art--although he soon left and Gene Colan took over from there (just as good but maybe more conventional).
However, I don't think realism or not has much to do with being cool. Comics art started out not being based in realism (unless you count book illustration by the likes of Albrecht Dürer). Most cartoon artists did surreal illustrations. You do have Hal Foster, Alex Raymond and Burne Hogarth doing art based on life--but they're the exceptions, not the rule. They were probably cool in their time because they went against expectations. The standard for comic strip art would have been guys like Windsor McKay, Richard Outcault and Rudolph Dirks.
And among comic book artists, it seems like a lot of them go betwixt and between illustration based on life and surreal depictions. Joe Staton and John Byrne at Charlton were doing "cool" art that didn't conform to reality--but when Staton came to D.C. and Byrne came to Marvel, their artwork became more realistic (although never completely). An artist like Neal Adams was considered the most realistic, but he was also considered avant-garde--the coolest of the cool. Keith Giffen tried out many different styles before he landed on his Muñoz style.
I suppose cool is what everyone is not doing yet, but soon will be doing. Trend-setters vs. trend followers.