Nick Cannon Says Mariah Carey Signing Rihanna's Breast Was 'Hot'

Nick Cannon was in the house to see his ex-wife Mariah Carey leave her mark on Rihanna's breast ... and he says more women should follow their lead.

We got Nick heading in to do some charity work Friday at the Los Angeles Mission and our photog asked him about the viral video from Mariah's recent concert.

Nick says watching Mariah autograph RiRi's boob was "hot" ... and he hopes it becomes a trend.

On a more serious note, Nick says it was really great of Mariah to leave the stage to go and greet Rihanna during the show ... and he gives RiRi some love for showing support for MC.

Nick's giving back to the community here for the holidays, and we also asked him what it's like to play Santa Claus for all of his kids, including the twins he shares with Mariah.

The children are getting older, which means the gifts are getting more expensive ... and Nick tells us if he's putting a limit on his Christmas spending.