There are some really important differences between Mundo/Garen and Poppy.
Mundo and Garen are juggernauts. They have very low CC and no team utility. If you they aren't doing a lot of damage and sponging up a lot of damage, they are completely useless. They are, quintessentially, stat checks.
Poppy has a ton of team utility. She has a small knockback that can stun, she cancels dashes in a large zone around her, and she has her massive ultimate knockback.
Poppy doesn't need to be as amazing a fighter in order to be useful. She brings stuff to the table aside from raw damage and tankiness.
Garen and Mundo need to be tanky as hell and do a lot of damage in order to ever be picked, because they bring absolutely nothing else to the table.
I do think Garen and Mundo can stand to have kit tweaks at some point. They are very binary and stat-checky. But in the meantime, they are just straight up underpowered, and could use some love.