Smoothen out the rest of the outlines, and apply anti-aliasing to the rest. If you want to go the extra mile, add another dark color to give it shading.

However if you want to do the other, darken the colors used for anti-aliasing a bit.

Here a quick tip of how you can also approach pixel art.
With black outline - With colored outline - with little Shading - and of course one without selective outline (which I don't have the time to do, but is basically breaking the outline - showing only outline on some parts)

Made some improvement suggestions (going from the model sheet I found online)

I hope I don't fuck up more