only when the hex specificly say so.
for example 'cusred wound' say "This is a curse effect and can be removed by remove curse". i myself would add any hex that talk about cursing even without naming it a curse effect.
but i'ts hard to call others, say like the flight hex, a curse when the only one it can effect is the witch herself.
Most hexes are not actual curses so will not benefit from the stubborn curse feat. RAW unless it mentions being a curse it is not. I do agree that if the description mentions curse it should be considered a curse. A quick check in Hero Labs shows about 15 hexes out of 110 that might be curses.
Yeah, by default, hexes aren't curses and don't profit from the feat But then again, they don't need the feat. The feat only affects the caster level check of Remove Curse (and similar spells if there are any), it has no effect on e.g. saving throws.
I would let the feat affect everything that can be removed by Remove Curse, because a differentiation between "curse effect" and "non-curse effect that can be removed identical to a curse" breaks the "things should be the same, or they should be different" principle (unless fighting linnorms or asura, maybe).