The irony of Shouko helping Karada with the birthday cake is that even back in her college days the adult Shouko was a helpless child at the kitchen and has not changed since then and pronounced more that chibi-Shouko's an adult who got her body turned into her child self.
Anyways, it doesn't matter if Shouko still harbors some resentment for Hiro after leaving her, she admits to herself that love for him still lingers on.
I think in the present Shouko desired to help in any matter possible to get that cake done, even if Karada did most of the work. She might wanted to amend to Hiro for not being a proper woman at the kitchen with that cake.
And the episode continues with the surprises. I understood that Karada would find out about Hiro x Shouko's affair.