My take is that Frankenstein is a surname, and, as Victor was the monster’s progenitor, they are both Frankenstein. If the context is clear enough for some pedant to “well, actually”, then it is clear enough to understand which Frankenstein is being discussed.
Personally I think it's ok to call the monster (or that style of monster) 'a Frankenstein' on the basis that it was created by Dr. Frankenstein the same way you'd call a painting by Picasso 'a Picasso'.
Yeah and Victor Frankenstein should be properly called Doctor Frankenstein, he didn't go to evil medical school to be refereed to as just Frankenstein.
Frankenstein -> the monster
Dr. Frankenstein -> the guy that made the monster