I think this was a nice concept, once I figured out how to fly, I enjoyed flying around and shooting at the monsters.

Conceptually, you have an interesting idea. Execution wise, there is much to be desired.

There is no clear way to convey to the player how to fly, let alone that you are flying. I did it by pure accident and thought I had perhaps encountered a bug at first. I also played the game twice because I wasn't sure if I had done something wrong when I died, or if that was legitimately just the end of the game. The sprites are fine, but confusing: The lack of any proper animation only makes it harder to discern what I'm doing, and the lava creatures... don't entirely look like lava creatures to me. I could certainly tell they were heat-related, however, so that's good. This probably goes without saying, but the lack of any music or sound effects really makes it feel flat. 

I think you did a great job with the i-frames, however! That's usually something most people overlook, so I'm glad you kept that in mind.

Overall, I would be hard pressed to say it was fun. It's just far too unfinished and unpolished for me to really gain much enjoyment from it, but I do think you're on the right track to really figuring out some proper mechanics for future projects. Keep it up, and don't feel discouraged!

I was confused how to fly at first, and a few times I died with no idea why.

It was fun flying around and shooting things. 

The game is a simple platform-shooter.

Good job on adding invincibility frames, and a game over screen.

But the game doesn't end if you remove all the lava creatures.

Also, you should clarify your controls, such as using the spacebar to fly.