As salamu alaykum, Is bulking allowed as means to gain muscle and weight rather than being skinny and does this go against the sunnah?

Yes it is allowed and being strong is good in itself, it does not go against the sunnah so long as it involves nothing forbidden and does not take you away from the religion. Eating past your fill is disliked though, even for this purpose, so build on eating to satiety and not beyond it.