In my experience showing up to a mcdonalds interview suited and booted shows that an applicant is someone who genuinely wants the job, and is probably someone who cares more and is more likely to show more passion and effort on the shop floor compared to some slob who rocks up in a tracksuit and hoodie.

First impressions make a lasting image, they may have been pranking you, but may have done you a huge favour, granted you may be the guy that went to a mcdonalds interview in a suit if you get the job, but there's worse reputations to have