If I had a problem with my Fitbit, would it be on topic to ask about it here?
Sadly I don't think these devices fit within SuperUser's scope.
As it is more an IoT device you might want to try Internet of Things. According to their help files
If you have a question about...
- controlling, automating and sensing the environment using electronics ('smart devices'),
... you're in the right place!
I would say that a FitBit is a smart device that senses it's environment. You might want to check on their meta first though.
Only if the problem was in connecting or using it with your computer.
Smartphones would not count as a computer though if your problem is with the Android application talking to it then there would be Android Enthusiasts or for Apple devices you could ask at Ask Different.
As Burgi mentions for the actual hardware itself there may be other sites on the network that are more appropriate. There used to be a "Gadgets" site, but I believe it did not make it out of beta.