None of the female characters of Buffy the Vampire Slayer is a Mary Sue. If you want to see a Mary Sue, just watch Star Wars and look at Rey who is killing off Star WarsLike most of you said no for Buffy and yes for Willow. But beside the way the relationship between the characters ended, there is a problem also in their personal development.
Buffy needed a Parker in her life as sad as that is. She was at a point in her life where she needed to experiment sex with someone else than Angel and to learn some harsh things about love and bad person. I think the Parker storyline made her stronger and in a way helped her to have a healthy distance in her relationships with man. With Angel she was ready to give everything for him and as romantic as that seem on TV it is not healthy. We can see her change with Riley because he wants all of her but she won't give it to him. Riley thinks she doesn't love him because he is still in this romantic stage of first love. But Buffy has learned that even though you are with someone you still have to stay trust yourself first and she learned that from her experience with Parker.
Willow didn't need a Kennedy at this point in this story. Willow had just gone dark and tried to kill all her friends. She had reconstruction to do with herself, her magic and the family she had built over the years. This is what she needed and would have been great to watch. Instead we had a sexy grilfriend distraction that didn't help her overcome anything special except to have sex with someone after Tara. If Kennedy had been a one night stand it would have been different but making her a steady girlfriend was a mistake.
Willow was shouting up to the sky, begging for Tara's forgiveness because Kennedy kissed her. That doesn't sound like someone ready to move on yet.To be fair (if you go by air date) Tara died in early May and Willow didn't really start dating Kennedy till February. That's 9 months. Not exactly jumping from one relationship to another. Plus I think, in the show's time, it was actually a little longer than that even. 🤔