The writing is also at fault, but Terry Farrell is also pretty bland in Becker.
I've always seen Terry Farrell as competant within a fairly limited range, but somewhat lacking in the capacity to stretch toward more extreme emotions the way Visitor and Auberjonois can, for example. I can't really rule out the possibility that the writers just never challenged her, but this kind of thing does seem to be reciprocal: the writers react to what the actor gives them and vice versa.
Odo to my knowledge was not conceived as a character whose soul was being devoured by romantic angst, for example, but Auberjonois could do both "gruff lawman on the outside" and "tortured adolescent changeling on the inside," and so the writers went with it.
Major Kira is tough and feisty as originally conceived, but once you see the vulnerable side of the character in episodes like
Battle Lines and
Duet, you end up seeing more of it in
Second Skin,
Ties of Blood and Water and many more, because Nana Visitor can be fragile without sacrificing her character's strength.
The characters end up having those extra dimensions because the actors have those dimensions, so when the writers go there, it works. Compare an episode like
Necessary Evil to an episode like
Chimera as far as character growth over time is concerned.
Not to exonerate the writers completely in the case of Jadzia (they don't handle her very overall imo), but Terry Farrell, while very likable, does seem to be most comfortable being, well... likable.