Dr. Fox was trying to invent a pair of underwear that would make sitting more comfortable, and she actually did! Unikitty obviously knows that Dr. Fox accidentally just made poofyer diapers, but she didn't want to tell her during her moment.
That or Dr. Fox purposely made thick padding, but Unikitty walked in on her and she just made up something on the spot so it didn't seem as she was making diapers for herself.
Also the joke with this picture is that Unikitty when she uses her arms, her body turns into Lego nub. So she's holding her diaper up so it doesn't fall off. Lol