I always thought he lacked in physical strength like Scar and developed a small body like him. Also when you watch TLK2 you can hear him complain about termites itching him all the time, So I figure he scratches at himself so much that his mane got really short and knotted from that and that the horrible treatment to his skin made his fur kind of unhealthy and the lack of water keeps him filthy and is what makes is fur look so grey. Plus Nuka isn't the brightest one ethier. I think he is just small and really dirty, Nuka is actually the Swahili word for "Stink"

P.S I just assumed Kovu was much bigger than him because of the training he had since he was little?

I don't see anything wrong with his face. But his mane is because of the breed of the lion he is. He is a Tsau (Don't exactly know how to spell it). Which is a breed of lion. Their manes are like that.

Well,look at Scar's body. It isn't like Mufasa's,Sarabi or any other Pridelander! And Kovu have a full grown mane and a Pridelander body,what makes him healthy. Nuka has a Scar-body and a messy dirty mane. His whiskers are also messy. That's why everybody calls him unhealthy.