Just tried it. It definitely evokes interest! It's one of those "hmm, I'll try just another coin. Right, enough. Ahh, just one more" type of games ;)
-Is the game too difficult?
I tend to say yes. But then again, I might just be too old for proper reaction times :P
-Is the resolution of the game too small? It's the first time I'm making anything that's not 640x480.
I don't think the resolution is too small. But you shouldn't let that bother you. I feel like this game will work in any size or resolution - as long as the levels are tuned to it.
-You opinion on the controls. Too sensitive? Too sluggish?
Too sensitive the closer you come to the ship. In about 60 pixels distance to the ship it feels comfortable.
-What about the graphics? It's not placeholder art, but if it looks really bad negative then I'll probably change it.
Don't change the style! It is clear, bright, with best contrast. The coins could benefit from a slightly darker color, and I think you could change the rocket to a paper plane (you are already blowing, so would fit), but all this within the current style.
If your goal is to tease the player a bit, then think about restricting the use of the "brakes". This is why:
I answered the first question by saying something about reaction times. In fact, I first had severe difficulties. Later, I just constantly pressed x and had a nice, slow-paced gaming experience :D