Is Finn a Marty Stu?
Let's see...
-Despite being a rookie Stormtrooper who raises the ire of both Phasma and Kylo during his first assignment, and being ordered to submit his equipment for inspection, he somehow has unfettered access, alone, to the most valuable prisoner on Kylo's ship. Are there no security levels for Stormtroopers? Why are Phasma and Kylo turning a blind eye to him directly after his incident at the village? There is also apparently no oversight with Poe; the questionable troop and the valued prisoner are able to simply waltz to a TIE fighter and get inside (of course that's a Star Wars thing, it happens on the Death Star as well with Luke, Han and Chewie).
-He's not familiar with ships yet he becomes adept at being a gunner so quickly that he can transfer those skills into the Falcon. He handles the Falcon's cannon about as well as Han and Luke did in the original.
-Despite his previous job in sanitation at Starkiller during its construction, he knows intimate technical details about how the moon-sized station works. It's like a plumber at a nation-sized Army base who somehow knows how to take down their security systems, even though he had never wanted to before now.
-Handles a lightsaber pretty well despite having never used one. Rookie Stormtrooper who uses it toe-to-toe with a fellow trooper. Sure he was probably trained in usage of that baton-like weapon as well, just like how Rey already knows how to fight with a staff since she has defended herself on Jakku for years.
-Without saber training or Force sensitivity, is actually able to hold his own against Kylo Ren, grandson of Darth Vader, for a short time. He even lands a hit!