After ending up as a lion-headed dinosaur, Simba ended up in a zoo. He's well taken care of but he doesn't want attention. Not while he, as he would put it, looks like a freak. Nick is there to give some words of encouragement.
As for why Nick was there in the first place? Well, Nick is a genuine, blue ribbon-winning cow. Only the best milk for the Dino-lion. Lion-dino? Lino!