As a rule black should never play f6 in most openings, but especially never the Kings Gambit, and Fischers Defense isn't weak, its just not Critical meaning that while it is perfectly sound white has plenty of quite reasonable ways to meet it.
Yeah it's not good for anything longer than a classical game but it should be sufficient over the board which is why modern theory likes black defenses that make the game as dry as possible
As a rule black should never play f6 in most openings, but especially never the Kings Gambit, and Fischers Defense isn't weak, its just not Critical meaning that while it is perfectly sound white has plenty of quite reasonable ways to meet it.