The Dane has joined for €18m as an emergency replacement for Ousmane Dembele.
"Honestly, I neither understand nor agree with [Braithwaite's] signing for several reasons," Rivaldo told Betfair.
"Firstly, because this signing was made outside the window and, in addition, because Barcelona could have chosen a kid from La Masia instead of [making] this transfer.
"I see no reason to make such an effort to incorporate a forward who, with all due respect, I doubt is the player that Barcelona need now.
"I was thinking of a younger player who could improve more."