This is a cool idea, but one issue I see is with the compositions of the buildings. Certain cosmetic packs may have blocks that take more or less time to break than the current designs, and players could get an advantage by selecting certain skins that have harder to break structures. Using your design an example, the time to break sandstone is 33% longer than the packed ice which is the best block to break on the current build. I don't think this type of objective advantage is something people would want to be unlockable via cubelet, and there may be other issues in regards to which player's cosmetic your team gets.
Even if cosmetics were designed with balancing in mind, it would substantially limit the possibilities for the designs of buildings, and the types of skins that would be available.