To add to what PrincessKiara said, I think Timon and Pumbaa are kinda cute. They have some cute moments at the very least. I mean just look at Pumbaa's big ol' puppy dog eyes here:
I wouldn't go so far as to say the hyenas are ugly, though the film doesn't exactly present them in a very appealing light, either. And I can't think of any moments they had that would be regarded as all that cute.
So from the standpoint of most parents, Timon and Pumbaa would be far more marketable. Parents would want their children to side with the good guys. I don't think the hyenas are evil, but I can see why parents may see them that way, or at least see them as undesirable. They sided with Scar, aided him in killing Mufasa and trying to kill Simba, and killed Scar in the end (it was justified but still). The only thing Timon and Pumbaa are guilty of is instilling a mindset in Simba that isn't
always right (though the wisdom of not getting overly-stressed about problems isn't without merit), and Timon and Pumbaa didn't know it wasn't always right, or at least didn't think they would have any huge problems to deal with given their situation.