It never occurred to me, when I first drew them in secondary school, that Oojo and Bink aren't wearing any clothes.
Cartoon characters that do wear clothes are mostly humanoid ones: The Simpsons, Charlie Brown, Super Mario, and so on.
Whereas a lot of animal characters don't: Snoopy, Garfield, Daffy Duck, etc.
Other characters wear one or two items of clothing only. Mickey Mouse wears a pair of red shorts, but no top. The Smurfs are similar; mostly topless. Winnie the Pooh does the opposite; he wears a red top, but no shorts or trousers. Which is a bit odd. And Sonic the Hedgehog runs about in a pair of trainers and some white gloves...
Anyway, as for alien cartoon characters, a quick image search shows that they're either naked or wearing some sort of space suit. So, there you go. Maybe at some point I'll introduce Oojo and Bink to clothes, but for now I'll let them shuffle about in their birthday suits.