Interesting that you pick displacer beast.
It has no resistance to nonmagic weapons, so the fighter isn't worse off than the wizard. In fact, since the displacer beast has avoidance, it's the wizard that's worse off because big spells like fireball/lightning bolt will do half or no damage. While the fighter is fine (AC 13 isn't that bad, even at disadvantage) and doing great if he took blindfighting (no disadvantage).
As for devils, the fighter is disadvantaged but not useless, half damage can still be a lot.
All that said, if the DM is doing nothing BUT throwing monsters at the party that disadvantage the fighter only? That's just bad on the DMs part.
The DM would ALSO be making a bad call to throw just monsters that screw over casters - of which there are plenty.