Good: the story is extremely cute, the FMC is extremely cute, the MMC's reactions are extremely cute. She's airheaded to the point it's unbelievable, but once you skip past that it's enjoyable and heartwarming.
Bad: there's an underdeveloped premise there, going back to your teens with the mentality of a 30yo. This premise would be amazing if developed, but as it is now it's just... dead weight, if you said "the MMC is 17, but he's really mature" it would be the same, you know?
Overall: a 6/10 for me. The story is enjoyable enough to keep going, but don't expect this to be the next big hit.