Really solid start! Getting something out there for a jam is never an easy feat, especially with so little time, so the fact you even made something playable in 8 hours is a huge accomplishment ^^
That being said, the UI covered a huge chunk of the bottom screen, and placing blocks felt a little slow with needing to re-click the button after placing something down. If you barely end up using the top of the screen for platforming, that might be a great spot to place your UI elements! I'd also recommend finding a way to either drag/drop blocks into the game, or have it so you can keep placing the same type of block as long as you initially selected it.
Otherwise, impressive work so far! Hope things go well if you decide to develop this game further ^^