Olaf just has a very strong kit which you cannot say about Mundo, Olaf benefits from going low in the jungle (passive + w) which makes or breaks him. If you can utilize it well you can clear as fast or faster (slower isn't possible) than the enemy jungler. If you can't, you'll lose the speed.
The Q and E are very strong for dealing damage, one being a slow and the other being true damage.
The W and ult are a perfect combo to survive in any team fight, w gives a lot of lifesteal and the ult makes sure your heals won't get interrupted by a stun or something like that.
Mundo doesn't have all these strong things. No true damage, only minimum. You use your health as mana, which makes jungling pretty difficult in the early. W doesn't really help, if you get stunned even for a second in pro play you may be caught out way too hard to get out. Ult isn't instant as lifesteal is, this means he can still be bursted down very easily.