It's not a matter of pride, it was invented and patented by Kraft and they marketed it as "American cheese" to try to get people to buy it. It's convenient for melting on a burger, but not a whole lot else, and nobody actually likes it except for weird freaks.
Mayo is fine, it's just disgusting in excess. You should apply it to your sandwich heavily enough that it moistens the entire surface of your bread slice, but lightly enough that there's little to no "standing" mayo on top of that.
Although not my cup of tea, the ingredient combination isnt alien, as mayo is just fat and eggs primarily, which along with chocolate, would be a core part of brownies (which you can sub mayo for ingredients with)