I must've missed that time period because i've never seen her act particularly dumb anywhere. From clueless in her debut, that obnoxious "ichiban kawaii" schtick in haromoni, to her sharp witty retorts in varieties she always had that knowing smirk and gleam in her eyes when she laughs/being laughed at.
If you're referring to her being bad at memory games/general knowledge though ... ah well, nobody's perfect

and i still think most of it are just the unfortunate typecasting of idols variety appearances in general as being not very bright
Sayu's little rabbits



Something about Kaneko seems ... off to me o__o