"Clean" means nice crisp lines surrounding the pattern with little to no dark speckling along the sides and belly.
"Dirty" will have lots of dark speckling or "noise" throughout the pattern and base colors and the pattern lines may be less distinctive.
Personally, I'm almost always drawn to the clean look. Like has been mentioned by others, I don't tend to find myself attracted to morphs and combos that intentionally dirty up the pattern, like granites or shatters. I also don't like many of the typical paradoxes for the same reason...they just look dirty and it's not attractive to me just because it's unusual. (although I have seen a few instances of extremely attractive paradox combos, these are not the typical paradox albino)