I like the monochromatic artwork. I think you did well with the pixel art (and I don't mean the opening art). Nice amount of content for a game jam. Three bosses is a good balance for a submission, I feel. I really like your use of the theme. Both for the gameplay as well as the story. The switching between different gameplay styles for each boss fight is also very nice.
The dragon was the coolest fight. The diversity in attacks made it quite fun. Few small gripes: the claw attack had a quick build up (for someone who's going to only play the boss maybe 1-3 times), inability to damage the dragon when they're on the side is unsatisfying (perhaps their tail could have persisted above), on occasion the dragon would spam the side attack (exacerbating the previous complaint). However, those are just small issues and overall it was quite fun.
One nitpick might be: the cursor didn't have to be visible (I think). Was kinda annoying to have on screen. Having difficulty options as well as boss skip options is a very nice feature in a game jam submission.
Overall, a very clean entry.
PS Hopping as the frog was unreasonably entertaining, though.