Comedy veteran Sheryl Underwoodsays the surprise “r-word” joke from Dwayne Johnsonduring Netflix’s “The Roast of Kevin Hart” worked because roast comedy plays by a different set of rules … and she says audiences understood the context.

Underwood stopped by “TMZ Live” on Monday and told us she thought The Rock’s appearance ultimately brought the room together … especially after years of discourse surrounding his identity and public image.

Watch the video … she tells us true roast comedy is not for the mainstream — acknowledging that some viewers may bristle at moments, but roasts have always been built around pushing boundaries.

“What I thought was good about what The Rock did, everybody was questioning his Blackness and he really made the connection,” Underwood said.

The longtime comic pointed out that being politically correct doesn’t always land the same way in comedy. Sheryl also defended the genre’s ability to tackle uncomfortable subjects head-on … saying humor can create common ground where regular conversations often fail.

“It’s a way for us to talk about the issues with humor and come to some common ground,” she explained. “Years ago, you couldn’t say ‘m-word’ for little people, couldn’t say ‘r-word’ for the disabled … sometimes political correctness does not fit in comedy.”

As TMZ previously reported, at the roast, The Rock dropped the R-word when making fun of Draymond Green.

Underwood also praised the overall vibe of the night … shouting out appearances from Usher and KattWilliamsto give Kevin an “elbow” along with his flowers proved the love in the room was genuine.

And she took plenty of jabs herself regarding her late husband — who died by suicide in 1990 — in stride … telling us that “freedom of speech is alive and well, and it’s alive on Netflix.”