It can help but i don't think it is as clear as in hitting. In hitting weight and strength almost always help power if mechanics are somewhat decent but being huge and strong doesn't always help throwing velo.
Some pitchers are huge and hulking and throw triple digits and others are thin as a stick and throw really hard. Some if the latin kids are 17, weigh like 150 with no muscle and still throw 96.
On the other hand frank thomas was huge and superthe powerful and barely managed to throw the ball from first to second.
But there are also pitchers who add 30 pounds and go from 86 to 95 so it definitely can help in some cases but not always.
There certainly isn't a direct correlation between strength and throwing, I think it is more like 90% of your velo is "natural" (i.e achieved with full growth, basic fitness, solid mechanics and some playing catch - which isn't quite natural of course) and the last 10% can come through training or not.
So a "natural" 70 mph thrower who deadlifts 180 (what I would call basic fitness) could go to high 70s if he improves deadlift to 350 and the "natural" 85 mph thrower who deadlifts 180 might improve to 92 with some strength.