So people think that her cutie mark is… fireworks? I don’t even think that she has her cutie mark yet! She became detached from society at a rather young age and nothing was mentioned of her gaining her cutie mark prior to leaving her home town (Source: The Stormy Road To Canterlot). Her life changed so abruptly, going way off of her original path, that I don’t think that she ever found her place in society. When she left, Grubber basically swooped her up and unless her cutie mark has something to do with being a commander for the Storm King, then I don’t see how she could have got it. However, it is indeed a possibility that she gained her cutie mark at the very end of MLP Movie when she shot fireworks in the air. Even if she did gain her cutie mark at that moment, Hasbro did not include that in the movie and if they intended for her to gain the mark,  
I believe that they would have shown it so we can feel happy for Tempest. So really, her cutie mark is up to the fan fictions to decipher what it is, or isnt. I think that some people can make very interesting and entertaining stories all about Tempest’s cutie mark and her place in society, whether she already has it or is looking for it.

I like this design for Fizzlepop’s cutie mark.