Bastion's probably got double to triple the dps of any other hero out there, but for the higher ranks he's too easily out-played and countered because of the better team coordination + he's slow, can't move in turret mode, and big hit box. In bronze, silver and maybe gold, you can play bastion and be pretty successful sometimes, but people will give you crap for playing it because he's not part of the meta.
Is bastion bad? Yea he's pretty bad for ranked and high-ranking games. I still like to play him in quick play for fun