Question: When people say "Unbelievable" in texting, does it usually mean "Unbelievably good" or "Unbelievably bad", or neutral, when the context is unclear?

Details about the background:

I send something neutral and playful to a friend, and the friend replied "unbelievable" without any additional words or follow-ups.

When the details are missing, how would you usually interpret the mood or attitude behind the word "unbelievable"? Positive or negative?

Answer: 
Without context, there is no way to know for sure, but my guess would be the more general sense of disbelief (“I don’t believe it”) regarding whatever they replied to.

For instance, if you said “my friend did [something stupid]”, that response would mean they don’t believe the friend could be so stupid as to do that thing.
In texts, it would *more likely* mean "amazing!" (ie, good) if the context is otherwise unclear.

In general in English, it's extremely common to use (confusing!) reversals, double-negatives, sarcasm as the norm, etc.