We wouldn't generalize about real people, so fictional ones should be treated no differently.
But isn't it pretty clear that we do generalise about real people? People generalise about real people all the time - watch the news, talk to people - so as part of an argument to not generalise in fiction it doesn't seem to work. (For me.) And people self-perform to generalisations all the time too - so generalisations aren't always from without.
Writers generalise a lot to get their point across -
Animal Farm,
Les Miserable,
Dracula... - all utilise archetypes to represent things.
Every
character produced is arguably a conglomeration of generalisations that exist in the author's head.
The problem lies in conglomerating(?) only one or two generalisations, all of which are clearly evident to the readers. Characters that 'feel real' are a combination of a much larger number of generalisations (usually with a very highly visible dominant one), so
feel less general.
So don't rest a character you want to be believable on a few generalisations. Make them more complex. But if you don't care if your character is believable, go ahead and use a stock character to illustrate your point. Just don't expect your audience to believe it is little more than a fable. But that
just might be exactly what you want.
gdm.