Why WOULDN'T it be real? I remember many years ago I saw a trivia fact that said around 50% of all restaurants in the United States had hamburger on the menu? Maybe that changed (it was a late 90s/ early 2000s trivia fact) but hamburger is still super common and popular.
Not to mention it's in a town called Hamburg. What city council that wants to get reelected wouldn't want a self-promoting festival called "Hamburg"er? Notice, they didn't call it the cheeseburger festival.
Man I just camped at Sleeper State Park in October, it's right next to Caseville, MI on the lake. That whole campground turns into a Halloween festival for the month, it's awesome. I bet those people do the cheeseburger festival right too. The only downside I see is I have to wait until Summer for it!
I will note though, that the cheeseburger festival isn't in Hamburg.