A mushroom cap is a slice or two of bread worth of protein and a good source of a few minerals and b vitamins while being low in calories. There are way worse things to eat (if you like them) if you weigh a twidge more than you'd like.
Which could be a plus: they're also VERY low in calories. And when it comes to a "burger" like Americans use, there's plenty of calories in the bun and if that's not enough, the amounts of sauce some people use.
It's going to be a lot healthier than a standard burger.