I give premades minor makeovers all the time. It's amazing how much a simple skin overlay and fresh hair and clothes can completely change a sim for the better. Some premades that seem a little "meh" are really just suffering from the wrong hair/clothes *cough* malcolmlandgraablooksamazingwithaminormakeoverandislowkeymyfavoritesim*cough*. Still haven't figured out how to fix Nina Caliente. Her face shape is stubborn.
I only give townies makeovers if I want my sims to date/marry/become bffs with them. Soemtimes there are certain townies that appear everywhere that I love and want to help out (there's this one DJ named Rieko Watanabe in Windenburg that I want to give a punk makeover and a barista named Theo Ball I want to look more polished). But for the most part I don't touch them.
edited for lack of basic spelling skills.