Koopa is a painfully average player. His power, speed, and fielding are just ok. However, contrary to Goomba, he fits perfectly on the Bowser team. His notable chemistry is with Bowser, Bowser Jr, and Dry Bones, so he’s a good piece to stick in there. What he offers to the team is a good star swing. We’ve seen how powerful teams need players who can get on base consistently. That’s exactly what Koopa offers to the powerful Bowser team, which is why he can have a big role despite his middling stats.