"Mutlu mular" is TERRIBLY WRONG. It is a mistake you may encounter time to time in daily speech by people who don't read much. If you make that mistake in writing, it will only show to the reader that you don't read at all or you dropped out of primary school!
The mistake is putting the plural suffix(-ler, -lar) after the interrogative particle(-mı, -mi etc.)
"Mutlular mı" is correct.