If you listen to music with a smiley-setting, you'll then try to mix it so it sound like that, when you mix on your non-smiley monitor system. Result, you'll get *two* smiley effects, one from your mixing, the second from the listeners stereo. Result: It will sound like shit.
So, you need to listen to music as you mix it.
So how should you mix it? Flat or with a smiley? Well, you should do your mixing flat, for one simple reason: Not everybody listens with a smile face, and a lot of music gets played on small crappy mono radios at work, and so on. If you mix your music with a monitor system set like a smiley, you will have no idea how it will sound otherwise.
Make your monitoring as flat an accurate as possible, and it is more likely that:
1. You will hear the problems with your mix.
2. That the mix will sound good on any system.