The F35 is primarily a air to ground strike aircraft. Gen 5 is mainly semi stealth benefits. It is disadvantaged relative to air to air specialized planes who are faster (lighter) and more maneuverable.
This is something you'll hear a lot, often from seemingly respectable sources, but it represents a fundamental misunderstanding of how modern air combat works.
Manueverability is meaningless. Dog fighting is a thing of the past. Stealth is not a disadvantage in air-to-air combat, it's the only thing that matters in air-to-air combat. The model has fundamentally changed; it's submarine / tank combat now. The winner is the person who sees their opponent first.
The people who describe the F-35 as unsuited to air combat are speaking from an entirely outdated understanding of how air combat works. Some of those people are even experts in that model of air combat, but that's like being an expert on vacuum tubes in a world of microchips.