So I've just been handed some ribbons at some random areas, and some hotels. Do they serve any other purpose other than "being there"? I'm looking around bublapedia, and serebii.net on what they are, but all they give is how to obtain them, and the description for each. It doesn't really say if it effects game play in any way. Though seems they've been around since Gen III, but I don't remember that back far.
My Greninja seems to be gaining the majority of them, as my first pokemon in-line of battles in this game. Though some ribbons that I've obtained multiple times I've gotten for the rest of my party. But I couldn't tell any gameplay differences.
As far as I know, they're just evident of the time of day (Pierce's Sleep Ribbon), level growth (Effort Ribbon), friendship (Mewtwo's Smile Ribbon), and the Footprint Ribbon. Just things to show how far your Pokemon have progressed.
And as for clothing... I can see a jacket for the humanoid Pokemon, and hats, but full outfits would be silly.