Lynne Marie Stewart, who played Pee-wee Herman’s perky, bouffant-wigged neighbor, Miss Yvonne, in the 1980s children’s television series “Pee-wee’s Playhouse” and the sweet, timorous mother of one of the main characters in “It’s Always Sunny in Philadelphia,” died on Friday in Los Angeles. She was 78.

The cause of her death, at her sister’s home, was cancer, said her manager, Bette Smith. Her doctors found a tumor shortly after Ms. Stewart finished filming a movie called “The Dink,” a comedy starring Jake Johnson and Ben Stiller, in December, Ms. Smith said.

Ms. Stewart played a variety of characters in a career that spanned six decades, and had nearly 150 credits as a screen, stage and voice actress starting in 1971, according to IMDb, the entertainment database.

But she was perhaps best known for her role as Miss Yvonne, or the “most beautiful woman in Puppetland,” in “Pee-wee’s Playhouse,” which ran for five seasons on Saturday mornings on CBS.

She was a fixture on the show as Pee-wee Herman’s extravagant neighbor with creative hairdos and a chipper personality.

With its whimsical and slyly subversive sense of humor, the show swiftly attracted an audience beyond its core demographic of preadolescent children, and Ms. Stewart and other members of its cast embraced its anarchic and surreal spirit of make-believe.