It really does mean she has been getting cuter, despite the present/future tense. The key is the note in the parentheses that she may get cuter in the future too.
Which tense you use with -てくる or -ていく depends on when the change starts and ends.
-てくる - became cute in the past, will continue getting cuter in the future
-ていく - became cute now, and will continue getting cuter into the future
-てきた - became cute in the past, stopped now
-ていった - became cute in the past, stopped sometime in the past
As you can see, neither past tense one will continue into the future, whereas both the present tense ones do. So if you'd used きた instead, it would imply that she would stop getting cuter now. Since the author meant (in the parentheses) that the action might continue into the future, she used the present tense to indicate so rather than the past.