Cinco De Mayo is not the same Holiday as day of the dead. Sugar skulls on Cinco De Mayo would be like showing Jack O Lanterns on the Fourth of July.
This is nothing new, they decorate Skulls for several holidays throughout the year (including in fact skulls decorated with sparkler hats around Fourth of July). Other holidays they might change the visual for normal Gems (e.g. shamrock Green Gem around St. Patrick’s Day).