Judging from these replies and my own experience, I would say that for a lot of people, things which go against the expectations of the theme do indeed detract from the 'fun'.

As with all things, it really depends on your audience. But if (for example) you're creating a Star Wars themed mud then I think it's fair to say that Star Wars fans will probably make up a sizable portion of your intended audience - and such people are likely to play with the expectation of the mud being relatively faithful to the books/movies.

I think as long as that sort of thing is ICly justified, it's acceptable - as in: if you're a human, you have to a) do a massive amount of weight training; b) practice lifting and swinging the sword around until you don't dislocate your arm when you try; and c) practice fighting with it for at least a few IC years, before you're even going to be able to lift it and wield it and fight - much less fight successfully - with that thirty-pound sword designed for giants.
It's when there's little - or no - IC justification that I object to stuff like that.