Will lifting make me a better LoL player?

No it won't Tyler.

Working out in general will help with blood flow, which helps with reaction time. Confidence also may help, although I'd think that's a minor effect. I'd suggest cardio over weight lifting if your goal is to work out and git gud at the same time. Or just do both. If getting better is the main focus, though, just practicing is gonna help more. Being confident doesn't mean anything if you don't know what you're doing.

Upvotes for 2 different answers!

As someone that lifts/exercises all the time, no it will not. Playing LoL makes you a better LoL player. This game's best players play this game almost all of the time, putting in a lot of effort to get that good. Also, lifting doesn't necessarily equate to a boost in confidence, especially if you're highly critical of yourself and/or hold yourself to really high standards.

Being a good decision-maker is more important than just being confident imo and they don't always go hand-in-hand. I suppose everyone is different, so maybe lifting will work for you, but you should really be doing it with the purpose to become stronger/healthier, not just to improve at a video game.

Exercise in general is good for brain health, and I could argue that lifting does it even more, but I won't go that deeply into it here. Look up some studies on Google and you'll find plenty.

It will give you the strength to pick up and carry shitty teams.