I am in favor of having the type of Npc influence the dressing code of Npcs. Also its important to remember than just because the apocalypse happened it means that everyone will become mercenaries/punks and that a lot of people would still dress and act with some resemblance of normality.
Groups could have the following flavor annd perhaps even safehouses serving as bases.
Gangmen: always hostile unless you start as a scoundrel or drug dealer and you are wearing gang colors. Wearing pants/cargopants, shorts, tank-tops, hoodies, bandanas and sunglasses, should be pretty easy to identify. The group is hostile to all others. Their bases are often low rent apartment buildings
Organized looters: could range from friendly to enemies unless the player is heavily mutated or dressed like a gang man in which case they are always hostile. They could wear from casual clothes to camping/fishing gear. Most often than not they should welcome you to their group if you have decent gear (either in good faith or only looking to kill you for your gear when you let your guard down). They bases could range from houses to shops to big shpping malls, omni-marts and complete neighborhoods
Rescue services: small possibly mixed groups of paramedics/police officers/firefighters should be always friendly unless the player is dressed as a gangman or is heavily mutated. They could take residence in public buildings (hospitals, police/fire stations, public works, schools, etc.)
And if we go the extra mile we could make buildings have chances of generating matching survivors near them, in both apparel and attitudes, instead of doing it like this. For example malls generating friendly people only doing the best to survive, shanty towns making gang members, high rise buildings generating organized looters, police stations and some houses generating lawful settlements around them, and so on and so on.