BT armour has to be designed to RESIST heat, since it's ablative and has to be proof against energy weapons that deliver heat. And thermodynamics don't really let you have it both ways, so in order to keep the lasers from easily melting through the armour, the armour isn't going let heat out the other way.
Is there an arguement that there ought to be a minor factor that as you punch more holes in a mech and blow bits off, it might cool easier? Sure, but a) that would be insanely complex and b) it could easily be such a comparitively small value that is doesn't matter. (Ditto the reverse for residualy heat from getting pummellled by repeated PPC fire ought to make it harder to shed heat.) Does that arguement break down a birt when you consider fire still heats mechs and that water doesn't evaporate a lot when a hot mech stands in it? Yeah, little bit, but you're then getting to the point of balancing a realistic thermodynamic situation without unknown technology or having an actually playable game.
As my Dad (a former aerospace engineer who did mathmatical modelling for a living) says; all simulations are wrong; some simulations are useful.