It does make sense that Finn isn't great with a lightsaber though- I believe the fluff for them has them as notoriously difficult to use, because they are very easy to accidentally chop part of the wielder off if they are not careful. Jedi mitigate this by being force sensitive, but other users are liable to injure themselves. With most weapons, you can feel what the tip is doing by it's weight, you don't have that with a lightsaber. It is probably why Finn does so badly in the duel with the Stormtrooper with the riot gear- the weapon would've felt like nothing he had ever used before, and he was probably trying to not chop himself too.
I think Kylo Ren's fighting abilities are fine in the last few scenes though- he has been hit by a flipping bowcaster round...