Fat isn't necessarily bad. It depends on the type of fat and the individuals caloric macro nutrient profile. The notion that fat is outright bad was conjured up decades ago and has since been shown to be false.
Some fats are healthy. However, I would not be surprised if the fats found in McDonald's are predominantly unhealthy.
To add to this, everytime I eat at Burger King I'm amazed at how god damn /salty/ it is compared to McDonald's. The saltiness isn't immediately obvious, but any day I have a a BK burger for lunch I end up drinking several huge cups of water throughout the day because of all the salt.
Over consumption of salt is far more harmful to one's health than over consumption of fat in general, as far as I'm concerned.
Some fats are healthy. However, I would not be surprised if the fats found in McDonald's are predominantly unhealthy.