Super cool game! Love the art in this, it all felt cohesive. I think the punching and dash attacks were cool, but I found that the dash attack was not worth using most of the time. I couldn't tell if there was more damage vs a normal punch, but I almost always got hurt when using the dash attack. 

If you wanted to push this project further, I think the powers ups would need to shake up the gameplay more. Getting another heart piece or becoming faster is useful, but I think different types of powerups and maybe different enemy behavior would do a lot for replayability. I think you could consider timed power ups too. In mario a star has a lot of weight not just because it makes you invincible, but also due to fact that you have limited time to make use of it. Same with the fire flower, you get stronger, but one hit and you lose that new ability.

In any case, I think you did a real good job on this!