Why is Zac hard to create skins for, again?

Based on what you previously said, I assumed the clothes would look bad - that they would stretch or warp in an unattractive fashion. But it looks fine (heck, it looks better than the distortion on Debonair Galio's collar when he dances...). Edna Mode clearly designed the outfit to suit Zac's particulars.

Admittedly, it would be preferable if you gave him custom chunks/blobbies for both parts of Cell Division, but aside from comedic value of having a couple trouser legs walking on their own - this isn't entirely necessary. But--based on the attached fan-made recolour--I honestly don't understand how creating a Zac skin takes more work than a Shaco, Heimerdinger, Zyra, etc, skin (in terms of having a few bespoke non-champion entities; heck, Special Agent Xin Zhao's recall animation has a brief case that required being made specifically...).

You could go one further and create a custom joke animation that shows off the skin (such as having fishies swimming around inside a water-themed skin) - but it doesn't look wrong having his outfit just disappear into his amorphic mass. My suspension of disbelief isn't flagging an issue (which was my perceived issue).