Mechs are cool, period!
I like that your Mech feels like a Mech or tank and not just like a scaled up character. In general the size differences are pretty cool. A mech should be huge compared to tanks, roads and other stuff. This works pretty well in your game.
It's also nice that you added voice over as tutorial, although you can easily miss them and if you move too fast you can trigger them in a way that they play at the same. But it's a nice detail.
To cool down your mech you need to stop moving and shooting which is fine, but I think you missed a really nice opportunity by using the water in the level as another cooling option. Like when you stand in a huge lake it should also cool you down and would give the levels a nice tactical feel because it would finding the right position to fight more important than just clicking as fast as you can.
Also maybe add some UI elements that show the player what the buttons 1, 2, and 3 do. I totally forgot which button I need to press to recharge the rockets.
I enjoyed playing your game. Good job! Keep going :)