If your Sim is appearing smelly when he's not, you could try the mailbox cheat. If you play on PS3, I think it's L2 button and X when selecting the mailbox, then some cheats will come up. There will be an option there that says Reset Sim, select that and choose the glitched sim. Hopefully that would fix it.
Are you sure it's your sim that is smelly, and not something in the house that is causing it (hidden mess, or rotten food in fridge, maybe some bad food in your inventory?)