The whole body is very stiff, try add some detail/movement (elbows, torso, legs) make them so that they aren’t in a stiff position for the whole of the animation.
The whole animation: The uppercut is in one position for the animation, this makes it look quite basic and simple.
It doesn’t look like an uppercut. It looks like the person is jumping in the air and spinning around with their arm raised. Usually people don’t jump when doing an uppercut but they do in anime’s etc so if its for that then thats fine.
Add some anticipation before the actual uppercut. Make it so that the person leans down a little with the spare arm by their chest then when they jump, the other arm goes behind them.
I apologise if this criticism seems harsh but I want you to create the best thing! Feel free to reply to this with your improvements!