Interesting concept. The music and sounds are lovely, and transforming feels juicy.
Design-wise, there's not much reason to spend a lot of time in "bat mode". You could spend most of your time as a vampire, transform into a bat for a split second to eat a zombie, then transform back. Maybe you could give the bat more abilities, like dashing, and make the bat transform time more limited.
Since it's a pixel art game, I'd recommend setting the texture filter options to "nearest", this would get rid of the blur and would make both the fonts and your characters look better.
Good job overall :)