Wow, this game is very cool! The art, gameplay and music are all very well done, and is generally very polished too! It also quite seldom see the upgrade menu being stylised and not just a bunch of plain text buttons. On that count, well done!
I'd say the game has yet to realise the full potential of the whole design, like for instance, I don't think all coins aquired during the level will be enough to buy all upgrades, and that's really a shame that the upgrade UI is wasted like this. And the upgrade itself doesn't really change up the game too much and just provides minor differences (except for the health) for the most part. The gameplay also feels a bit harsh with a small explosion radius verses fast moving enemies, especially the last green tall mushroom, which I don't think I am able to defeat it without taking damage.
Buy anyway, this is a very nice and polished game, and I like it very much! Great job!