In context of the anime series
Sailor Moon, it's a reference to the title character's hairdo. An odango is a Japanese dumpling.
Usagi's
boyfriend Mamoru often refers to her as "odango atama" or "dumpling head".
In the English dub of the anime, he calls her "meatball head", but in the English translated manga, he calls her "Buns", which might be sort-of accurate but is really kind of perverse when the subject in question is a fourteen year old junior high student.