I haven't read "Hogfather" (though I've read a few other Discworld novels) so had no idea there was a "pretentious suburban snob" character with the name in that book.
I must say that I think most Americans would have more of a "rural hick" image of
Twyla than "suburban snob", unless they are fans of
Twyla Tharp, the famous dancer and choreographer. However, I note that the Wikipedia article on Ms. Tharp says she was born on a farm in
Indiana and claims "She was named for
Twila Thornburg, the "Pig
Princess" of the 89th Annual Muncie Fair". Yup, the "Pig
Princess" at the Muncie Fair fits what I assume is the American stereotype of the name. :)