I noticed that in most of my matches, I'm top frag after like round 5, but end up bottom fragging by the end of the game, and I don't know how to fix it. Is it something that happens often to people?

It feels like I'm playing League with an early game champion that stomps hard early, but then ends up getting outscaled easily (which I think shouldn't be the case in an FPS game). I play Viper if that's relevant, and I only started playing last week. Is there some sort of skillset that comes in mid game that I'm lacking? I was thinking an issue may be that I use my Q less often than my ult (due to me not having an idea what to do with Q, E is just bigger and they drain the same fuel anyway), but I don't think that should matter much in unrated games vs other new people.