Much cleaner for sure =)
The robot's right arm and the armchair-looking 'arm' below are now parallel and of similar length: I'd suggest to change the robot arm to 3x1 or 4x1 clusters.
There is still a lot of banding (see link in previous comment).

As for 'charm', I don't know exactly what you mean. I now see that it has a face (hadn't noticed before): you might make it more expressive.
Also the general design of the robot is kind of weird, it looks like a robot upper half grafted on an office armchair. There is no clue as to why it's like this, how (and if) it moves, what it's meant to do. If all these questions had a visual answer somehow, the piece would definitely be stronger.
Better consider this as a program for the next piece though ;)

Oh, here are some changes. First I tried to fix some shapes and perspective problems then redraw lines and AA, time to time adding some details. I decided to practice with ellipses a bit more then . After all I think this robot had lost his charm /Maybe because his chest is now wider and he looks 'harder'/ Anyway, what do you think?

Oh thank you, master . Your experience is really helpful. Now I'll try.

Edit:

I polished the outline, mostly in the stand and wire, also a few other sore spots (arrows). Some others remain. Tbh I used the ellipse tool for the stand base to save time . With some practice it's possible to do the same by hand! Using the ellipse tool to check is ok too, be sure to have the correct perspective however!

I kept the black outline option you started with, but its limitations quickly become obvious in thin and small parts like the wire and plug. It's customary to not AA the outside of sprites for bg integration, however you have to ask yourself if this piece is indeed a sprite, or a self standing piece which could make use of a static bg and allow for better AA.

Whenever possible, try to stick to 'natural' pixel angles for smoothness, like I did in the arm (2x1 angle).

It is important to check your work permanently at 100 or 200% (the intended viewing scale), when you work at higher scales for comfort you lose the overall view on which smoothness and other factors depend. If your software allows, open 2 windows of the piece (large and small). If not, zoom in and out for almost every pixel you draw!

Don't be afraid to mix the colors from different zones, the piece will only look better for it. Your palette is not well balanced, many colors are very similar while there are gaps between values elsewhere, these make it hard to AA with.

Fixed a few other things here and there: a perspective issue in the upper torso, and lots of banding. None of these fixes is exhaustive, up to you now ;)

Maybe if it's easy for you you can give me some advices. When I do this I try to keep the line width and direction. Now I've got no idea what I should do to make this better.

I prefer the version without outlines, it fits better the realistic shading which is quite good.

But both versions have very jaggy linework / borders: please clean up! It's really an easy fix in the worst affected areas.