How much of the outer Solar System could you see with the naked eye if you were on Mars, providing Mars was the exact same as Earth, only smaller?

It would be pretty much the same view that you would see from Earth. The Sun would be a little smaller, and the Earth is a bright dot in the sky. You also wouldn’t obviously see Mars in the sky. But otherwise it would be the same.

Consider too that what planets are visible has most to do with where the planets and sun are in relation to each other, and which way your own planet is facing when you look. Some orbits take a very long time to complete. The difference between the orbits of Earth and Mars tend to be less than the difference between the outer planets being on one side of the sun or the other, for example.

There would be fewer distortions from the atmosphere, which means the stars would “twinkle” a lot less and Jupiter would be a little bigger (brighter), but the differences should be otherwise negligible.