Well, sort of. It’s actually a Costume Mario form unlocked by beating a NES Remix inspired level, and grants Mario no extra abilities whatsoever.
So in other words, a Frog Suit in name only. None the less, here’s a video showing the suit being unlocked in the game (recorded just a few hours or so ago):
And here’s another video, this time by Youtube channel GameExplain, which shows the suit in action:
As you can see, it’s definitely only a visual thing, with no real effects on gameplay (as best seen in the underwater area). But what do you think about it? Is it a good idea that this Mario form was added as a Mystery Mushroom thing, or would it have been better if the powerup was added as an actual powerup complete with the abilities of the original one from Super Mario Bros 3? Personally, we think the latter.