Well done on your first animation!
Overall I kinda like it as it is. It has a certain charm to it. I imagine a game where everything was drawn in this style could be fun and engaging to play.
The background works. It has a low contrast and high colour values, which makes the higher contrast, lower colour value helicopter really stand out.
I like the way the helicopter is never still. This gives a bit more interest and action to the scene.
However, you want to improve and there are some more principles you could employ.
Firstly, you have mixed the perspective on your helicopter. The fuselage is seen exactly from the side and the blades are seen from the side and above. Now, you could just call this your style and do the same thing with other elements in your game. It's kinda cute. Or you could go completely in the other direction and make them horizontal. One example of that I've seen recently is "Cure hunters"
https://www.youtube.com/watch?v=FWx3oc6X7MEIt's very quick at the beginning of the video.
The other thing about the helicopter rotor is that it's moving too slowly. The blades in Cure Hunters are moving too slowly as well, so again, you could just call this part of the style and carry on. But there are ways of making them look like they're moving fast.
A (very) quick search yields this, which probably does a better job of describing a way of doing this than I could. It's for Flash, but you could easily apply the same principles to pixel art.
https://youtu.be/IqosfImdfqMIt's also using a mixed perspective, but not quite as extreme as yours I think. You can see a similar technique in action in Doodle Jump.
https://www.youtube.com/watch?v=w3POq5jNjT4&t=13The Flash video also makes the point that you need a tail rotor.
For animation smoothing, you could try adding some techniques like "squash & stretch" and "anticipation".
Here's a quick video about anticipation:
https://www.youtube.com/watch?v=pTFmluAvr24You could try moving and tilting the helicopter back a little before it moves forwards to add anticipation, and then give it a more obvious acceleration as it moves forwards.
Squashing and stretching are a little more difficult to apply, as helicopters are hard objects. If you look at animations or videos frame by frame though, you can often see some bizarre or unexpected single frames where things seem to stretch or squash. If the helicopter was made of rubber you could have it squash to the left in anticipation, then stretch out to the right as it accelerates forward. As it's made of metal you may not want to do that.
If you wanted to use a cartoon-like style then you *could* do this kind of thing, and it's a common enough technique that I've seen it built-in to at least one platformer game engine. (For a main character jumping or landing for example.) If you want to keep the solidity of this hard metal object, a "squash" anticipation phase may or may not be great, but you could maybe have a single frame of "stretch" after the anticipation, as it starts to move forwards. Think of it as "motion blur".
I feel like you could have some more obvious "easing" (often called "slow in" and "slow out"), or deceleration and acceleration as the helicopter comes to a stop and moves forwards.
Techniques like this can help give some life to an animation. Too much and you're in sploopy, squishy cartoon world, but add just enough and should find your animations become a little more engaging and believable, and thus seem smoother.
So as I said, your background works, and I like the effect of the sun. You've got some shape and volume into those sand dunes too. However, it could be a bit TOO low-contrast and too light. The danger is that you could end up with the background looking a bit anaemic. Maybe you could make the dunes into their own layer and add some more layers in front?
This game uses a lot of different backgrounds (content warning: pixelated violence and gore and stuff):
https://youtu.be/ph5fVkkv6QAAs the layers get closer to the viewer they have higher contrast, more saturated colours. The mix of distances and palettes makes the whole scene a bit more immersive.
A final point on the background. Despite my avatar, I'm not a big fan of large amounts of dithering. Specifically here, it can tempt you into decisions that give angles and hard edges where there should be curves. On the left, where the dithering blends into the darker shade of sky, is one example. This is just my opinion, but unless you're working with a specific palette to emulate old hardware, you have access to any colours you like.
Some subtle dithering (unlike my avatar) can add texture and warmth to an image. Too much can be a bit overwhelming for me. On the other hand, it's a valid technique, so it's down to taste. Use too many colours and you can end up with a painterly style, which has the potential to look less like pixel art and more like pixellated art. And there is a difference. Things like dithering and obvious edges between shades in a gradient can help define pixel art as an aesthetic. Dithering is a tool, find the path that works for your style.
Anyway, I hope there's something in there that you find useful, or at least interesting.
