A good question.
I had considered this previously with an earlier sketch. The plates would be made of fire when excited and the thagomizer would be funky looking. However, going with your suggestion of seeing a Stego-Poke with Chlorophyll because of the various proposed uses for the plates as a use for thermoregulation, it made more sense for the plates to be solid permanent features rather than flame in origin (plus it seemed too much like I was taking from the Cyndaquil evo lines). It's also the Stego's most distinctive feature, like Pachy's Skull, or Raptor's sneer, or even Spino's spine. Having it be a temporary structure seemed to... be a waste and didn't fit with it having Chlorophyll.
Besides, what's wrong with it being a branching evo Rock-Fire?