Check out the full list of Super Mario Maker 2 achievements/Mii costume pieces

In Super Mario Maker 2, you can receive special costume pieces for your Mii if you achieve certain goals. Some of those items will be quite hard to unlock, and almost unobtainable for some. Still, if you want a complete rundown of what you can get and how to get it, check out the details below.

Uploading your first course: Fish Bone Shirt
Getting a like from a course you made: I-Like-You Camisole
Getting a comment from a course you made: Blockstripe Shirt
Having your course get 100 plays: Shy Cap (Looks like a Boo)
Having your course get 500 plays: Cheep-Cheep Hat
Having your course get 1000 plays: Bouncy Shirt
Having your course get 2000 plays: Running Shirt
Having your course get 5000 plays: Kuribo Shoe
Earning 2000 Maker points: Staredown Shirt
Earning a lot of feedback on a course you made: Antsy Corduroys
Earning a lot of feedback on (another?) course you made: Fried-Chicken Hoodie
Liking a course for the first time (Multiplayer VS/Co-op doesn't count): Googoo Onesie
Posting your first comment: Banzai Bill Shirt
Clearing your first course: White Tanktop
Playing ten courses: Slobbery Shirt
Clearing ten courses: Skull Skirt
Playing one hundred courses: Cat Mario Suit and Cat Mario Headgear
Clearing one hundred courses: Big Spender Shorts
Playing five hundred courses: Thwomp Onesie
Clearing five hundred courses: Propeller Mario Hat and Propeller Mario Outfit
Playing one thousand courses: Mario Cap and Mario Outfit
Clearing one thousand courses: Toad Cap and Toad Outfit
Being the first person to clear a course: Parent-and-Child Skirt
Being the first person to clear a course 10 times: Hot Hot Shirt
Being the first person to clear a course 100 times: Chomp Dog Shirt
Setting a world record (it counts if you're the first person to clear a course): Pipe Hat
Setting ten world records: Pipe Skirt
Setting one hundred world records: Mario Swim Trucks
Earn your first bronze medal (top 300 on a leaderboard): ? Block Hoodie
Earn your first silver medal (top 200 on a leaderboard): Rocky Wrench Manhole Lid
Earn your first gold medal (top 100 on a leaderboard): Koopa Troopa Suit
Earn your first bronze medal with spikes (Unlocked by getting 3rd place place in a leaderboard category): Matrimony Dress
Earn your first silver medal with spikes (Unlocked by getting 2nd place in a leaderboard category): Fancy Tuxedo and Fancy Top Hat
Earn your first gold medal with spikes (Unlocked by getting 1st place in a leaderboard category): Royal Attire and Royal Crown
Getting a high score of 10 courses in Endless Challenge (Easy): Bowser Jr. Headpiece
Getting a high score of 100 courses in Endless Challenge (Easy): Stingby Skirt
Getting a high score of 10 courses in Endless Challenge (Normal): Angry Sun Shirt
Getting a high score of 100 courses in Endless Challenge (Normal): Faceplant
Getting a high score of 10 courses in Endless Challenge (Expert): Platform Skirt
Getting a high score of 100 courses in Endless Challenge (Expert): Cloudless
Getting a high score of 10 courses in Endless Challenge (Super Expert): Hoverclown
Getting a high score of 100 courses in Endless Challenge (Super Expert): ???
Winning your first Multiplayer Versus match: Shorts of Doom!
Winning consecutive Multiplayer Versus matches: Bowser Headpiece and Bowser Suit
Winning five Multiplayer Versus matches in a row: Fireworks Shirt
Getting C rank in Multiplayer Versus: Luigi Outfit and Luigi Hat
Getting B rank in Multiplayer Versus: Doctor Outfit and Doctor Hat
Getting A rank in Multiplayer Versus: Laughing Shirt
Getting S rank in Multiplayer Versus: Nintendo Uniform
Getting S+ rank in Multiplayer Versus: Superb Suit
Clearing your first course in Multiplayer Co-op: Burner Skirt
Clearing ten courses in Multiplayer Co-op: Magikoopa Hat and Magikoopa Robes
Clearing one hundred courses in Multiplayer Co-op: Yoshi Hat and Yoshi Outfit
Finishing Undodog's jobs in Story Mode: Reset Dress
Finishing Partick's jobs in Story Mode: Partick Shirt
Finishing Mr. Eraser's jobs in Story Mode: Refreshing Shirt
Finishing Soundfrog's jobs in Story Mode: Frog Cap
Finishing Yanamura's jobs in Story Mode: Yanamura Shirt
Hitting a ? Block in Story Mode (a specific one that can be found by clearing Purple Toad's jobs): Superball Mario Hat, Superball Mario Suit, and the Superball Flower powerup
Complete "Meowser Showdown" job in Story Mode: Builder Mario Hat, Builder Mario Outfit, and Super Hammer powerup
Finishing Princess Peach's first job in Story Mode after completing the castle: Princess Peach Dress
Finishing Princess Peach's second job in Story Mode: Princess Peach Wig
Finishing Princess Peach's third job after giving Toadette 10000 coins in Story Mode: Princess Peach Tennis Outfit
Completing all jobs as Mario: Robot Suit and Robot Head