I'm a bit late to this conversation, but I have a theory as to why 'costumes at Halloween should be scary' seems to be the rule in the UK.
In the US we don't have a culture of fancy dress parties (I never was invited to a costume party of any sort (other than on Halloween) in the 40 years I lived in the US. So the one time of the year when you can dress up means anything goes! I was never anything scary for Halloween, ever. The fact that it has to be scary would have really cramped my style! This is not to deny that there is a real scary element to Halloween in the US--of course there is! I won't go into the whole Samhain thing, most people know and can look it up on Wikipedia if they don't, but that is the origin of Halloween. But scary does not mean evil, but won't argue that point here.
Anyway, growing up in the 70s and 80s in Philadelphia meant bags and bags of candy, most to all of the neighbors getting in on the action with great decorations and loads of kids trick-or-treating, dressed as whatever their hearts desired.