It felt really odd that the author neglected to mention the reason the stunt should have fallen flat was that he was using it to mock Harris for actually having worked at McDonalds, not cosplaying at it.
That's the part that seemed odd for an article trying to put McD in the spotlight of the recent events. It just read as completely random "look, how crazy he is, he decided out of the blue to stage a McD cosplay foto session".
The stunt worked because there's some controversy over the ability to verify Harris worked at a McDonald's as she stated. Trump very publicly worked there for a day to sort of rub it in. He was trolling her, and it appears to have worked.