Pipotchi has a long list of things he wanted to do if he was somehow turned human.
He cannot think of any of them right now, because he's so distracted by how cute Jimmy looks.
Jimmy just lets himself to be cuddled by Pipotchi and wear the cute little outfits.
(Jimmy also thinks that Monkey!Spike is adorable and is a willing accomplice to Pipotchi putting Spike in cute little outfits)
Jimmy's strength should be multipled by being an ape, but no one can tell the difference.

Consider the following: Pipotchi and Jimmy both now have height on Spike, who unfortunately by getting species swapped with Specter got a whole new DNA scramble from the hybrid and is much smaller than either of the two who want to dress him up, and while Jimmy would fit in most of Pipotchi's clothes, the options to dress Spike are limited to Large Doll clothes and baby clothes

Chimpanzees are known to prey upon smaller apes, so Jimmy already has a serious advantage over Spike and can keep him under wraps no matter how desperately Spike tries to escape his frilly fate.
Pipotchi has no qualms about dropping a chunk of change on cute little outfits for Spike, even if he'll never get to use them again.
(Imagine his surprise and delight when they come in useful for Loo years later)

... I both knew that and forgot that about chimpanzees...

Yeah Spike rerolled the worst potential genetic combination off Specter's DNA list if he wanted to avoid a chimpanzee overpowering him and putting him in clothed he does not want to wear. It would be hard enough to escape alone without having to cope with the toe-thumbs while he runs

Spike has never had to endure this before, being an only child and older than his cousins.
He has had to tolerate Specter putting him in dresses, but that didn't have the indignity of his cousins fussing over him like a newborn, so this is a new depth of humiliation.

Also from the perspective of Pipotchi and Jimmy,
Pipotchi has never had something that small to fuss over, While Jimmy hasn't had someone that small to fuss over since Pipotchi was a baby so this is an opportunity that reasonably would not show it's face again for any known amount of time. With Pipotchi becoming less dependent as he ages Jimmy could very well fall into an empty nest syndrome as the one thing he felt he was particularly good at was being Pipotchi's brother and caretaker. This behavior would be standard fair for Pipotchi but for Jimmy to go along with it would be unexpected but not un-understandable as he is basically team mom without a baby.

Replace characters with AE characters
https://www.youtube.com/watch?v=8q8eFwPPJ4U