I think that it makes people feel good and therefore it’s a big Mitzvah (as long as they are the same gender as you or a little boy).
It doesn’t promote superficiality. If the person was not superficial to begin with, it won’t make them become superficial – they will just be happy that someone cares about them enough to say something to make them feel good. And if they are superficial to begin with, it won’t add to it. People are superficial (in this way) out of insecurity. By complimenting them, you help them to feel more secure.
The only way that I could see it encouraging superficiality is if you only compliment expensive items worn by Jappy people and never compliment people who are not Jappy.
If you are concerned about encouraging materialism, why not make it a point to compliment inexpensive items worn by non-materialistic people that are nice but not expensive?