Nope, level matters somewhat (they just have access to more weapons with different stats), what's matter here is your skill and choosing a weapon that can fit it.
When I first played Splatoon 2, I also chose a Roller, played Inking-every-single-corner role and was in your position, but after grinding through Singleplayer mode with some other weapons, I decided to switch to Dualies, started playing aggressively, got more victory, and less time being at the bottom of the board (sometimes I went top, woohoo!).
Don't know about anyone else and it could happen to just me, but I think it worth a try.
tl;dr:
- Go through Singleplayer with some other weapon types to see what fit you more.
- Try changing your playstyle.
- Choose weapons that has stat, sub-weapon and special that you feel comfortable with (test them out in Sheldon shop).
- Oh, and start buying and grinding your gears, scrub 'em when they full, save up abilities, think about your character build and build your gear toward it. These things matter.
- If you're still losing (like standing at the bottom of the board, otherwise, blame your teammates) when you have the best and fittest gears in your hand, then it just the matter of skill, go do more battles and git gud.
Also, try watching Youtube for tips and asking Gamefaqs, reddit, etc, I'm sure there are others can give you a better advice.