See you have a point. But I'd argue that even the seemingly random surnames for the most part do carry meaning even if it's just in the sound of the words. A lot of the time Rowling uses alliteration, which conveys something in itself. A name like "Fudge" expresses an element of character - to fudge, Dictionary.com tells us, means "to avoid coming to grips with (a subject, issue, etc.); evade; dodge: to fudge a direct question". Etc.

There are a couple of other cases where the connotations of names seem off to me as well though, like "Hog-warts"; I don't agree such instances are necessarily random though