Is it Acceptable to Refer to the Monster as "Frankenstein"?

It is because the Jason character literally isn't the killer in the first movie and people are dead wrong. The analogy is like saying that people think the name of the mother is Jason. Now I agree that many people think the name of the monster is Frankenstein, which is part of the marketing. Just look at most of the posters. It is Frankenstein with a big picture of the monsters face. The one movie that makes it clear is the James McAvoy movie Victor Frankenstein. I will say that the Peter Cushing movies do make a bigger distinction.

There is a place for being pedantic and I don't think the name of the monster is that place. In conversation the name Frankenstein is goijng to come up either way, you need to say his name first when referring to the monster or people aren't going to know what monster you are talking about. I think my argument is apt, though I sort of understand your POV. Arguing over whether Jason is the killer in the first movie isn't nitpicky. It is language vs. an actual character. These people think Jason has always been the killer and that is a much bigger issue.

The difference is that I know the name of the character isn't literally Frankenstein and I think my POV has been made quite clear about why I think it is acceptable to call him Frankenstein and I don't think I am being unreasonable. You even point out how you even posed the question of whether the monster has ever adopted the name and that seems to leave ambiguity even in your mind.