considering that the tanks that have a high winrate are those who specifically do well against AD comps that shouldn't be a surprise, now tell me about the rest of the tanks.
I have rarely seen
being built in games where Mundo was in so... yeah, kinda easy to not be shut down when people rarely build the item... and even then when it is built it actually has a pretty significant winrate, now imagine if it was built more often against the champions it's supposed to counter.
Kog'Maw is only underperforming because you never reach the late game, most games are over way before he is able to do anything as opposed to MF who is a lane bully that is strong in the early game and would struggle the more the game advances but since the games rarely reach that point of the game... it's no surprise