I have a couple of issues with the game, mainly with the controls: right now they are too floaty for a precision platformer like the first SMB, I think that a new player controller based on ray casts rather than unity physics will improve it a lot. Can't comment on the art style as they are a free asset here on itch.io.
Overall there's potential, but need - IMHO - a rehaul of the player controller.
Viewing post in Jumping Jive jam comments
If you use Unity, there’s this awesome series of tutorials by Sebastian Lague (https://youtu.be/MbWK8bCAU2w) that explain the theory behind it! It’s a bit old so some coding may be obsolete, but the principles are solid!