Nope. My first casting of Time Stop taught me that overwhelming application of magic can go overboard. 5 DBF spells going off at the same time did enough structural damage to collapse the entire cavern we were in. The cleric barely had time to Plane Shift us out.
It's a totally fine ruling to say it's flammable. I'm just saying the spell itself doesn't say it is. I think treating it like Alchemist's Fire once ignited is a good house rule. (1d4 damage at the start of each of its turns and can use an action to attempt to extinguish with a DC 10 Dexterity check.)
Nothing wrong with that. It doesn't specifically say it isn't. Even if it did house rules are fine so long as everyone at the table enjoys it. I think Grease being flammable is fine. It's a pretty minor change and it is how many people think it acts anyways.