Compare the critical hit table location by location: a biped mirrors its arms, its side torsos and its legs, a quad mirrors both pairs of legs, and a design is symmetrical when every mirrored pair holds the same things. Every slot counts rather than the weapons alone, so an actuator removed to make room, a lone heat sink and the Endo Steel a design spreads through spare slots all break it. Nothing that is not a Mek is asked: a tank has a left side and a right side and no rules-level notion of mirroring them.