Some people think that she's important just because she's a love interest.In TW1 she's definitely more important than in other two games,but her role is still pretty minimal.She's important in the prologue and act 3 and barely present(or not present at all)in other acts.Characters like Alvin,Shani,Yaevinn,Siegfrid,Kalkstein ect have just as big if not bigger role.In TW2 Triss spends more then half of the game captured.And in TW3 she has almost the same amount of screentime and importance as Keira Metz.
In tw2 I was much more interested in capturing Letho because he kidnaped Triss than because he killed Foltest (even though I liked Foltest)