Most clothing is unlocked by getting A Rank in the Hi-Score Derby modes. All clothing can be color customized by using the shoulder buttons (by default it's your Mii's shirt color).

Cloth	How to unlock
Basic Uniform	Get an A Rank in Bat & Switch's Hi-Score Derby A.
Business Suit	Get an A Rank in Bat & Switch's Hi-Score Derby B.
Caveman Suit	Get an A Rank in Gear Games' Hi-Score Derby A.
Football Uniform	Get an A Rank in Volley Bats's High-Score Derby B.
Kung-Fu Outfit	Get an A Rank in Cage Match's Hi-Score Derby A.
Manager's Jacket	Get an A Rank in Gear Games' Hi-Score Derby B.
Ninja Suit	Get an A or A+ Rank in Bat Master.
Pro Ump Outfit	Get an A Rank in Make the Call's High-Score Derby A.
Rusty Slugger Jacket	Purchase all 10 of Rusty's games.
Sweat Suit	Get an A Rank in Cage Match's Hi-Score Derby B.
Tropical Shirt	Get an A Rank in Volley Bats Hi-Score Derby A.