Not every attempt to improve a mosque is an act of boasting. Purpose and conduct matter: are resources being used for worshippers' genuine needs, or mainly to gain prestige and outshine others?
يَتَبَاهَى has a verb pattern that here expresses mutual boasting or rivalry. The concern is not simply that an individual does good work, but that people compete in self-display.
Good maintenance, service, and order remain valuable. The warning asks a person to resist arrogance rather than make unwarranted judgments about the hidden motives of a particular congregation.