Wow, so many people really thought even for a moment the ganguro was going to win?
The MC is a insecure teenage loner, that he started to grow.
After his first success (the cheer squad) he was put into the outcasts group, destroying the little bit of fragile confidence he got in the festival
He was just going to say yes because he thought he thought he had no alternative, he won't get another chance.
Of course he is going to change his mind after knowing that that another (and best) girl, with whom he is more emotionally involved, likes him.
The timing was horrible, but the reaction is understandable.
Her reaction is understandable too, she legit thought he was going to say yes, after saying she's cute and shit.
Still better than 90% of manga MCs.