Thats not the reason why i call fierce deity evil.
The reason is simple, in the actually game, if you press the A button on the firece deity mask, the description will say something like "This mask can only be used in boss rooms because of its evil power"
in the game, if you press the A button for info on the firece deity mask, it is saying something like:
"this mask can only be used in boss rooms due it dark power"