地味 can also mean "soil fertility", which matches her perfectly.
Another possible translation is "modesty".
Both translations fit onto Sachikos character.
By the way, there are a lot of plain characters in manga, but mostly not in romantic comedies and/or if they exist inside, they usually only are minor sub-characters at best.
That's because people won't buy manga with plain-looking women inside unless there is a good story (which about 90+% of these kinds of manga are completely devoid of). Even Shoujo manga nowadays only have beautiful female characters in their cast, even if the theme is supposedly "reverse beauty and the beast" (i.e. "ugly" female & "beautiful" male).