Note that you can also unlock these outfits during gameplay, but scanning the corresponding Amiibo will allow you to unlock their outfits right away.

Outfit	How to unlock
Bowser Groom Outfit	Use the Bowser (Wedding Outfit) amiibo
Classic Mario Outfit	Use the 8-Bit Mario, Smash Bros Mario or the Mario series Mario amiibo.
Diddy Kong Outfit	Use the Smash Bros Diddy Kong or Mario series Diddy Kong amiibo.
Dr. Mario Outfit	Use the Smash Bros Dr. Mario amiibo.
Gold Mario Outfit	Use the Gold Mario or Silver Mario amiibo.
Luigi Outfit	Use the Smash Bros Luigi or Mario series Luigi amiibo.
Mario Groom Outfit	Use the Mario (Wedding Outfit) amiibo
Peach Bridal Outfit	Use the Peach (Wedding Outfit) amiibo
Waluigi Outfit	Use the Waluigi amiibo.
Wario Outfit	Use the Smash Bros Wario or Mario series Wario amiibo.