As you said, it does take some time to get used to the movement. I think making the movement feel better and adding a few more features would allow this game to really shine.
Overall, I liked it. It's enjoyable early on but interest fades fast.
The arm's artstyle feels so out of place (maybe that was the intention), but I think a simple to implement solution is to make the entity shapes not filled in. Or in other words, just outlines. That way the lineart of the arm matches a little more. Maybe in actuality that would cause the arm to blend in too much, would need testing.
I know things are supposed to increase in difficulty, but the powerups don't make enough of an impression or make the later game feel any different. An unfortunate consequence to this is that I lose the feeling of being an overpowered arm slingshot of death as time goes on.