I think she might be yes..with the matching lockets and all..or maybe she was the one who helped him cut his heart out..
The one thing that could put an end to the Tia/Davy coupling theory is the question of how old the legend of Davy Jones is? I mean it seems as if he has been around much longer (generations) before Tia was born..Tia is not exactly old in this movie. Maybe it was Tia's mother or grand mother who was his fiance..and the locket has been passed down to Tia.
What also interests me is that she seems to pick up on the fact that Will's destiny with Liz is going to mirror Davy Jone's tragic love story...which looks bad for poor old Will 🙁