Base Game Not Required | Available until Jul 5
Base game not required to claim. Free to claim until July 5th (source). It's called a pre-order bonus, but it's just claimable for everyone so.. yeah.
Playstation:
NA - https://store.playstation.com/en-us/product/UP1023-PPSA11540_00-WFLIFEDLC0000005
EU - https://store.playstation.com/en-gb/product/EP4492-PPSA11541_00-WFLIFEDLC0000005
if you have problems using above links, try this regional free search link (first result): https://store.playstation.com/search/story%20of%20seasons%20a%20wonderful%20life%20mukumuku%20outfit
Xbox: https://www.xbox.com/games/store/pre-order-bonus/9pcl52msjb25