A very solid entry for the first game ever. Nice little platformer puzzle game. Very intuitive and brings a cool shape-shifting mechanic. Also, the art isn't as horrible as you think haha, you undersell yourself.
In terms of core mechanics, I'd suggest making the shape-shifting have no cooldown. That way it would make it snappy and more fun to engage with. And of course, more levels and complexity, later on, would also be appreciated. One last thing I've noticed there are some bugs with the hitboxes, sometimes I couldn't move at all when there were no obstacles or even with the appropriate shape I couldn't pass through the doors. Just something to look into in the future.
Good job!