Stewart Snyder isn't in this book for very long: he's basically just a dude who knows Carol in college and tries to get her to marry him. Carol basically thinks of the guy as "a competent bulky young man in a gray flannel shirt" (1.1.13). Now, we know Carol will never be happy if she marries Stewart. The truth is that the poor guy "was not interesting. He hadn't a shapely white neck, and he had never lived among celebrated reformers" (1.1.21).
To be fair, Carol does go on to marry Will Kennicott, a guy who doesn't turn out to be much more interesting that Stewart. But that just seems to suggest that no matter what Carol does, she's going to end up with a boring husband and become a housewife. That's kind of depressing, but it's also the sort of thing Sinclair Lewis wanted to critique.