An ethical discussion? For what it's worth, I understand what you're saying but I think this is more an issue of semantics. "Good but overrated" reads to me like "good, but not amazing or all time great."
There are series that I genuinely think are overrated, but I would not attach much praise to them. In the case of Gunbuster, ...if you watch it without context you could be excused for saying it's pretty cookie cutter, generic and dated. I certainly felt that way the first time I saw it, which was about 4 years ago. It's kind of like anything great from previous eras in the sense that once you accept it for what it is, you see just how special it was and how much attention went into making it.