It seems that Super Mario Maker 2 will be missing all of the costumes from the first game in the series, as it has been confirmed that the game won't include amiibo support and that the Mystery Mushrooms won't be making a comeback.
The original Super Mario Maker had items called Mystery Mushrooms that could give Mario a costume based on different characters. It was possible to unlock some of these costumes by completing in-game challenges, but some of them could be unlocked straight away by using an amiibo. There were over one hundred and fifty costumes in the game, which included the likes of Bowser, Wario, Mega Man, Pikachu, Link, Ness, Shulk, Tom Nook, Sonic the Hedgehog, Fox McCloud, a Mercedes-Benz, a Felyne from Monster Hunter, the band BABYMETAL, Shaun the Sheep, and the Duck Hunt dog.
The people over at the GameXplain YouTube channel were given a chance to play Super Mario Maker 2 and they discovered that the game won't include amiibo support, nor will the Mystery Mushrooms (and their costumes) appear in the game. They confirmed this information with a Nintendo representative at the event.
The lack of amiibo support and the additional costumes seems to be a huge step back from the original Super Mario Maker in terms of content. One of the best things about the Mystery Mushrooms and their costumes was that it allowed players to create stages themed around the different characters. The players weren't just defined by Mario in Super Mario Maker, yet the loss of these costumes and their unique sound effects means that the scope of the game has been limited.
Super Mario Maker had one of the best applications for the amiibo of any Nintendo game and the absence of amiibo functionality in Super Mario Maker 2 suggests that support for the toys might be fading away. The lack of costumes in Super Mario Maker 2 is a major step down from the original game and it seems such a shame to lose them when the game is receiving so much new content in other areas.
Super Mario Maker 2 will be released for the Nintendo Switch on June 28, 2019.