As such, "on fire" is a pretty bad aspect in the first place. Once you set someone on fire and they take damage from it, that's usually it for them, there's not much further you'll go from there. And that looks like a taken out result to me. Going for "singed robes", "burning jacket" or something more specific like that would probably be easier to work with. Especially since it also gives an indication how the aspect is best removed.
I tend to go either or in these cases, not both. This goes back to the idea that Fate is trying to emulate fiction rather than physics. The fact that someone is "on fire" and they don't get any stress doesn't mean they don't get any damage, it just means that at this time in the story, this specific damage doesn't matter. It makes more sense when you look at it through the lens of a movie or tv show rather than a way to emulate how real fire would work. Like when two people are fighting and suddenly someone gets his jacket singed and he might not even notice it until he gets attacked with a barrel of oil that breaks apart above him, drenching him in oil and because his jacket was still flickering, he is now completely on fire and taken out of the conflict. If he is still in, he might not be on fire, but jumped quick enough so the oil was still ignited in a large ball of fire, but he got away with just a scratch and some superficial burns.
It's basically all about how you phrase things. If you say "well, that might be a bit much", you might be doing a bit much for the action you are trying to do.
You could fractal aspects like that out, making them characters of sorts on their own. I'd actually started to create some sort of environmental characters like that a while ago, I'll have to see if I can still find it somewhere.
The idea basically boils down to this: You create the aspect "on fire", which in turn creates the "fire" character" with a single skill: burn. Depending on how severe the fire is, the higher its skill. It can attack after that, but it could also just spread around the room.
However, this can drain the spotlight away from the characters and give you a game that's pretty focused on such environmental hazards.