"He's a really smart guy, and Jessica really loves him," the rocker, 31 – and husband of Jessica's sister Ashlee – told PEOPLE at Monday night's VH1 Save the Music Gala in New York.
After all of her romantic ups and downs, Jessica's love affair with the Yale-educated former NFL player is a pleasure to behold, Wentz added. "I think that's the most important thing, seeing her happy," he said.
Wentz has spent time with Johnson, 31, and seems to have a budding athletic rivalry with him. "I think it's wonderful that when we play tennis, he does not absolutely slay me," Wentz joked. "No, but he's wickedly good at tennis."
Wentz also weighed in on wife Ashlee's new blonde pixie hairstyle. And guess what? He's a fan: "I love her haircut," he says. "She can wear her hair any style, any color. … If I bleach my hair, all of a sudden I look like Charlie Brown."
The new cut accentuates one of his wife's best features, he added. "Her neck's one of my favorite parts of her body," he said. "Not in a vampire way. She's got a nice neck."