Well, yes, of course they're stereotypes, as are all the characters in the book, they're real people but pushed a bit further in the direction of a caricature, in the same way that Aziraphale is pushed towards the effeminate bookshop owner, and Newt is the lame would-be geek, etc. But still, you have to actually observe people before doing a caricature, that's what makes a good one...