Completely agree. It depends on the genre. If it's a fighting fantasy, probably one of the last things on the readers minds is what outfit is the character going to wear to this battle. But if it's a slice of life, then the reader would expect the characters to behave more realistically and change their clothing regularly.

Yes, it is very dependent on the genre. However, I think a middle ground is best: minor changes in appearance can be frequent, but more significant changes should be attributable to major changes in the character's life (e.g. passing of time, personality change, some kind of physical or spiritual transformation etc.). I expect characters not to show drastic changes if it's only a one volume manga, though (unless it's because of one of the major life changes I mentioned).