It can be used, but I cant argue enough that you shouldn't. Even with a 50% increase it still starts of with a measly 55/50 Atk/SpAtk stat. so the actual effect of Light Ball is really not that big. Add to that a shallow movepool, predictable sets, an average Speed of 90, and a myriad of better Pokemon with better stats even withouth the Light Ball. It really dont need any longer explanation than that, Pikachu is a terrible Pokemon and it is rubbish competitively.
It must be mentioned that it can be used in the lower tiers, such as PU, but it is still not any better than mediocre, at the very best, as its sats are still subpar and there are still plenty of sweepers that outclass it.