You can select cloth combinations from the shop but that's it. If someone has some special hat or costume they might have gotten it in some past events.
if by custom clothes you mean can you design outfit pieces to be used in game, yes you can, using the templates & information in this guide threat by WarViking. If they're good enough they may well be added to the game- the majority of the hallowe'en hats and some of the gym leader outfits were designed by players.