I liked Keiko in general but I agree she didn't come off well in the 'Relationship trouble' episodes. But I don't interpret it as being a 'bitch' at all, I see it as just being strongly opinionated and having a strong sense of self, which are two qualities which are considered positive qualities in men but are labeled 'bitchy' when they appear in women.
...or, is it possible that most of her dialogue was written by male(s) from a male's perspective? I don't know about you but I always found myself rooting for Miles in those situations, and I'm pretty sure the writers arranged it that way. In episodes where she appeared, Keiko was often the antagonist to Miles in the B or C story. I'm sure most of it had to do with the fact that Miles was a "main" character, but I wouldn't be surprised if male bias was part of it as well. I'm sure the writers being mostly guys brought their marriage experiences to the table when writing these episodes. Of course if you're rooting for Miles, you're going to be annoyed by Keiko. Having said that, I didn't dislike
Keiko. I found the introduction of these sorts of real problems (and joys) that everyday people and couples can relate to refreshing for Star Trek. Perhaps humanity has evolved in the 24th century, but men and women still don't understand each other!