mundo is designed that once he is tanky, he can 1 v 1 almost anyone but he can't do much against a group in terms of damage so he just takes chunks of their hp at at ime
for instance, doesn' tmax rank Q take 25% of their current hp and does a set minimum of up to 300 I think
but overall, mundo's just fun to play as an is an actual tank...he's not op or anything. other tanks do way more damage and just tank a bit less
zac for instance can knock 20% of everyone around him's hp every few seconds so long as he has 500 ap, which is easily reached
aatrox can duel almost anyone once he is tanky
chogath is one of the least tanky of tanks but does a ton of burst damage (and unlike other tanks, has trouble against tanks)
gragas chunks up to 20% hp but his entire kit was nerfed so that's all he's got
singed and ekko are the only champs that make it where chasing is the worst thing you can do against them
rammus is almost immune to physical damage and has a built in thornmail that stacks with thornmail and 1 of 3 taunts or so in the game
mundo is a very good champion, but he is perfectly balanced, maybe even a little less than balanced