For a first game, awesome job. The first game I ever tried to make was the worst version of Tetris you will have ever played. Horrendous garabage messes are VERY common in first games, and they are learning experiences. This is NOT that kind of first game.
Did you do the graphics yourself or get them from somewhere? If you did them yourself you need to stop being so down on your work, they look amazing. I grew up on Atari and early NES games. Game designers had to convey A LOT with very little at that time and your little people convey everything they need to. If you're not confident enough for more complex models in future projects keep the retro style and just look up some of the top rated old Atari and Arcade classics. I think you could make some really neat retro things with what you can already do.
The colors for the world are a little, bleh, but the mummy itself makes up for that in spades, because the bleh colors make the mummy pop out more and so your eyes are always drawn to the mummy which is the focus of your game.
Are you using the built in physics from game maker? I honestly hate them for simple things. It's actually pretty easy to make your own for something like this just using a few variables. If not, then I agree with everyone else here and just tweak what you did a little. The acceleration on forward movement takes too long on the start up and you spring up way to high, way to fast, every jump. That said WE don't know what your intentions were for the movement, and you don't have to follow any advice. It needs tweaking, but everyone here has different ideas on how to build a game and what we want an end product to be so following us wouldn't make it YOUR game anymore if that makes sense? Tweak it however you see fit.
As for difficulty, I think it's fine. Whether it should be harder or easier is up to the scope you were looking at for your game. How long did you expect people to play your game, when, and why. Google has this little dinosaur you can play whenever you lose internet connection. The game itself is technically pretty bad, but it meets it's scope perfectly. Internet is stuttering, and it gives me something to do while I wait for it to stabilize. It's basic, memorable, and only needs to go a few minutes because that's all the time I really have for it. I love that little dinosaur game. This game is similar it's something I can do while waiting for a longer game to load or laugh at my family/friends trying to play it while we hangout.
One request, though, is to have some music. Things are a little too quiet playing the game. Otherwise fantastic job, I'm going to be laughing at people flipping the mummy into a pit later.