No need to dress to impress this holiday season!

Katy Perry** and boyfriend John Mayer **looked downright comfortable -- and casual -- snuggled up together during a dinner date in Beverly Hills Thursday night.

Perry, 28, ditched her usual glam, skin-tight dresses for a pair of oversized spectacles, army green jacket and colorful scarf, while Mayer, 35, kept things chill in a blue-and-grey ensemble.

The couple, who spent Christmas with Perry's parents up in Santa Barbara, Calif., kept their heads lowered as they ducked into Matsuhisa for some sushi and conversation.

Aside from a brief split this past August, Perry and Mayer have been inseparable since first stepping out together five months ago.

And the relationship is quickly getting serious, friends of the couple say.

"John is surprised at just how cool and smart she is," Mayer's friend told Us Weekly.

Another pal added that "John has proved he's matured and learned from his mistakes . . . Katy is blown away."

Earlier this month, the pair -- accompanied by Mayer's father, Richard -- checked out the Broadway adaptation of classic 1983 film A Christmas Story in New York City, where Perry introduced John to the cast as her "boyfriend."

"They are seriously in love and exclusive," another friend told Us recently.

More proof?

On Christmas Day, the "Firework" singer posted a photo of Mayer dressed up in Santa garb for all her 30 million-plus followers to see.

"Santa baby," she captioned the snap teasingly.

This article originally appeared on Usmagazine.com: Katy Perry, John Mayer Dress Down For Beverly Hills Dinner Date