It really isn't objectification. Objectification is meant to be degrading and demeaning. She's just a horny adolescent who likes tits. Also, the other girls said she doesn't like her own body, and that it isn't so bad having someone obsess over it. It's flattering. There, it's been addressed.
It's not that simple. A lot of the time Harumi-san is treated as the thing that is attached to her boobs, and that's objectification.
I think the protagonist might eventually realize/reveal that she likes Harumi as a person and not just for her boobs, and that would be healthier for this series, I feel.