The phrase "be dizened" means to be dressed up or adorned in a showy or gaudy manner. Its antonyms would be "unadorned," "plain," or "unembellished." Other antonyms could include "undecorated," "unornamented," or "unostentatious." These words suggest a lack of flashy embellishments or showy adornments. They imply a simpler, more subdued appearance, free from the over-the-top ornamentation associated with being dizened up. Overall, the antonyms of "be dizened" all suggest a more subdued and minimalistic style, one that prioritizes simplicity and functionality over extravagance and showiness.