Fat isn't the greatest reason McDonald's is looked at as an issue. I know nothing about you. You may eat incredibly healthily. American's, as a whole, do not. Nor do they get enough exercise. Enjoy your quarter pounder though. You are not the average American, clearly.
This is why it's best to reserve comments that stereotype groups as a whole. Bet if someone had made a comment targeting a specific ethnic group, they'd be slaughtered. But target a nationality, and it's A-OK!
Example: Ethnic group XXX sure doesn't need to eat anymore XXX!
Result: Flame fest
Example 2: Americans sure don't need to eat anymore McDonalds!
Result: Laughter and nods of agreement