ASSUMING you read the text you'll know that the customer was TWICE informed that they don't serve what she ordered.

Customer asks for a product.
Customer is told they are not served in the "fat dispensary place" she walked into.
Customer gets mad because she can't order a product not on the menu.
Customer obviously is dumb*** who didn't care about finding out what they served, insteading acting like a ***

What I don't get is the last line - she either knows he's kidding and that's a stupid thing to say, or she's supposedly too stupid to know he's kidding.

Hate to be the captain obvious here, but what the actual ***ing hell does any of that crap have to do with the joke in the picture here?

Does the picture, in any part anywhere, imply the cosplayer doesn't know who she's cosplaying? No it ***ing doesn't.

The joke is __OBVIOUS AS ***__ intended to be on the idiot who added the white text.

TL;DR: you are very stupid and you'd probably be better off elsewhere.