it's because snoke is the guy pulling the reins. basically, snoke is the true leader of the first order, and kylo ren is kind of like the teacher's pet who gets a lot of power simply because he's snoke's favorite. if you look at the script, it's obvious general hux hates his guts because kylo ren does whatever he wants and hux sees it as an absolute nuisance. when he's unhappy with kylo ren's decisions, he goes and reports directly to snoke instead of hashing it out with kylo, which shows who he sees as the legit leader of the whole operation.

i have no quote for that, since official discussions of the film after it was released are far and few (we just have all the interviews with the actors and writers being tight-lipped about the details of the film), but just based on the writing, acting, and even character design, he's supposed to be underwhelming. for example, darth vader would have never had a scene where the audience was supposed to laugh at stormtroopers reacting to him, and yet kylo did (in that tantrum scene), and that's definitely intentional.