Hmm, most of my sims haven't caught fire making those herbal remedies but the very first weenie roast my legacy family did was, in hindsight, hilarious as nearly every guest caught fire at some stage as well as three family members (one of which was the next heir). I don't think I've had a sim catch fire just roasting anything, but to be honest I play so many game files in succession that the details become a bit blurry!
I do know that if you use the interaction 'add a log' too often, or an uncontrolled guest does, then a fire is pretty sure to happen. lol. One easy way to kill a sim by fire, add loads of logs to a fire pit.
As for the herbal remedies, I'm currently playing a father and son who are in Granite Falls at the moment. Neither of them have caught fire at all during their skilling up and the father is now L10 and has completed the corresponding aspiration and the son is not far behind at L9. My guess is that traits do have an impact rather than skill, but I can't prove that, it's just a hunch.
As a side issue, when throwing the last 10 and 15 social events for the party animal aspiration, you can just throw it, arrive and then end it immediately. I guess it's a kind-of cheat or an exploit but if it's just a fun game and not for scoring/points etc it might be handy to know.