They're*
revise "just as worse at"
Actually, I think it's jungle mains that are most often full of themselves. They describe themselves as infallible because it's just someone else making the mistakes in lane.
"You just gave him a kill, way to go."
"Not my fault you fed him, he's your problem now."
"He has only a 10 CS lead, and hasn't even backed from the kill. You're the one that decided to jump on him while he was two levels higher and I couldn't participate in the gank, pinging you to stop."
"Shut up scrub, I'm the jungler, not you. I actually know what I'm doing."
^Legitimate conversation that's repeated itself several times^
Marksman mains can be full of themselves on occasion, but they're actually pretty cool about things for the most part. They'll say "my bad" frequently, and say things like "dis [insert support here] is a god".
Rarely do they have no idea how to appreciate the support, and that's when it goes downhill, because they think they can 1v2 just because they got fed by your hand, which did literally 90% of the work, and they've just been last-hitting champs.