Yes, fire can disable modern military vehicles. Fire can disable pretty much everything else in the world too. That still requires a lot of fire to get it that far. I also imagine that the heat from your flamethrower isn't going to be entirely effective right away. It'll take time to get that reactor into meltdown (which, I honestly don't think you can do that since Gundams use reactors but they're like 17 meters tall but for the sake of the RP it's fine) which means you need to keep the flames going as long as you can. If the enemy starts skating away, you're going to be in deep shit since the speed of both of you moving is going to make the flames behave a bit weirdly. But the whole thing about it here is that heating something up takes a lot longer than to be deemed necessary or effective. You will be slow-cooking these FRAMEs instead of BBQing them like you want, and that's considering they stand still and don't shoot back at all.