It's because Archers are probably the worst class in GBA Emblem, if not the entire series. This is made even worse in FE7 because effective weapons only have double might as opposed to the triple they usually get, so Rebecca's not even that good at killing flyers. She cannot compete with anyone who use Javelins, Hand Axes, or magic.
Add onto this the general backlash against Waifuism. Because she's a cute girl she gets popularly points from some for the whole "waifu" factor. Others push against that resulting in the apparent hate for the character. It's not dissimilar to how parts of the fandom turned again Lyn, execpt Rebecca is even worse of a unit than her.