Another user mentioned this on reddit, thought it worth sharing for those that picked the game up.
If you sign up to the Super Monkey Ball newsletter you get sent a DLC code for the platform of your choice, unlocks 3x yellow coloured hats to dress up your character with. A couple of caps and a top hat.
Should be noted that if you’re not keen on signing up your not missing heaps, same cosmetics are available in game normally just different colours and designs. Unless yellow is your favourite colour I guess. Yellow top hat is pretty dapper to be fair.
I signed up last night, got a confirmation email a few hours later letting me know I’d receive my code in the next 10 business days, but got it emailed through this morning sometime.
Unsure of when the promotion finishes.