I don't see how giving people less options in their appearance is beneficial. Sure, some people will make their outfits look ridiculous, but they'll do so with anything. The monk outfits and Justacorps are nice items mechanically, but if they don't fit the themes many people go for with their characters, they won't use them. I've bought monk outfits that looked great in the store, only to find I'd be permanently stuck as Cammy from Street Fighter. My monk is an Indian-style monk, so more of a Dhalsim. That doesn't mean I should have to track down another monk robe with different attributes or whatever that looks like Dhalsim - just letting me adjust the original robe would be enough.
Same with the Justacorps. If there are in-character uniforms, people could just alter whatever they wore to look like the uniform - no worries.
At the very least, the clothes should have a disclaimer if they're unmodifiable for some reason.