It's obviously not viable to implement literally every single design imaginable straight in CSS. If I want to do something silly like placing my content on the faces of a hypercube it wouldn't be realistic for me to expect that to be part of CSS. Having me write some JS is completely acceptable - that's what it is for, after all!
It's not a matter of "would this be better in CSS", the answer to that is painfully obvious. The relevant question here becomes "is this common enough that it warrants a CSS implementation". And honestly? In the case of Masonry I am not convinced it is.