I am the complete opposite. I like that they change their clothes. I like to mix it up a little.
This is one of my favorite little touches in the game. It's going to be sort of a love/hate thing since it's a touch of realism that has impact on gameplay (players cannot as easily keep a consistent look on someone), but I think it's a small thing that ends up adding a lot to making the characters feel more genuine.
I'm often amused by what the characters decide to wear, e.g. switching to Michael and seeing him waking from bed in a panicked startle, drinks on the floor, and then cutting to him emerging from the closet wearing a suit as if thinking "man I gotta get my shit together."
Ton of nice little character touches in the game. I love how they incorporated much of that into the character switching as well. Franklin will often be hanging out at his house enjoying the digs, playing with Chop, or getting plastered at a bar or strip club, generally conveying a more youthful, exuberant attitude. Trevor will be doing some random crazy thing - that impressively always feels in-character - or living it up in squalor. Michael will be doing things like driving along listening to moody music (Phil Collins), or quietly hanging out near the beach having a cigarette, being more a lonesome, introspective sort.