I don't at all mind the idea of Surtur having a daughter. Just not Sindr. She really is just a flat, nothing character. Made without any understanding of what Surtur is. It's just like, "hey here's a big ol' evil fire demon girl! You like that, right?"
Now if Surtur had a daughter that was a real answer to Odin having Thor, a true dark goddess of cosmic fire, that was the champion/defender/representation of Muspelheim, I would be behind that idea in a heartbeat.