The word translated as "build" has the metaphoric sense of "build up" as in making something bigger and more important.
The Greek words translated as "garnish the sepulchers" and "decorate the graves" actually means "arrange the memorials." The biblical translators make it sound
While the word translated a "garnish/decorate" has a secondary meaning for "adorn," its primary meaning is "to order" and "to arrange," a verb form of the word that Jesus commonly uses to refer to the "world order."
The word translated as "sepulchers/graves" means "memorial" or "record." It is a noun form of the verb meaning "to remember." This could refer to the role of religious leaders giving speeches at funerals or creating other forms of memorials for those who were seen as particularly good examples.