The thing is, if we just look at this from a semi-realistic perspective, flamers would be ineffective weapons in mechanised warfare. They would have about the same effective range as shotguns, but be way, way less damaging.
And from a gameplay perspective, heat management is already a complicated affair, with the various dashes and every attack costing heat. If the weapon is to be balanced, it cant cause a total overheat(from 0 to 100+) in 1 or even 2 shots, because that would cause serious issues for the player(since they often end up being outnumbered at higher stakes fight, and getting overheated would draw out the fight), but on the other hand, it does very little direct damage and needs to be at close range, so high risk, low reward, since it wont kill anything fast.
As for elemental status effects in general, I dont think they really add much to the overall gameplay. The more mechanics you add to the game, the more it needs to be balanced with everything already in the game, and needs to be validated in such a perspective. It needs to offer the player something interesting other than the novelty of having it, while still keeping every other option interesting and engaging. And it needs to be balanced in terms of giving and receiving.
I am not saying it is impossible to add, but it is another layer that needs a lot of attention to work right, and I dont see it add much to the overall gameplay.
There are better ways to add status effects than elemental attributes in a Mech game.