Never tell Delia Deetz that she cannot live and breathe art. She will go insane and take you with her. It's not a pretty picture.
And, uh, neither is her art.
Delia's character is how the filmmakers get to express their not-so-subtle opinion on the 1980s avant-garde art scene. See, Delia's your classic cliché of the artist snob. She's only happy when she's sculpting, but it's clear she's pretty awful at it. While we might admire some of her interior design and creative fashion choices (a headband made from gloves, a sweater doubling as a skirt), her sculptures are pretty grotesque.
It's enough to almost make you feel bad for Delia. Almost. But when it comes down to it, Delia's too self-centered to get anyone's sympathy. Even though her family clearly loves the house they're living in, she's determined to do an extreme makeover to impress her New York friends.
She's not much of a mother figure, either. We don't know what happened to Lydia's biological mother, but Delia isn't exactly jumping in with the warm fuzzies. When Lydia meets the Maitlands and tries to tell her stepmother that they're living in a haunted house, Delia's not receptive:
DELIA: Ghosts. You're telling me we have ghosts in this house. Those pictures are of ghosts. Are they? Ghosts! Lydia, I am giving a dinner party tonight for seven people. My agent, Bernard, is bringing a woman who writes for Art in America. In fact, no one dining here this evening has not been in Vanity Fair. Except you.
LYDIA: I told them you were too mean to be afraid.
DELIA: Don't you dare speak to others about me! The only thing that scares me is being embarrassed in front of the few hip people I can get to set foot in this part of Connecticut. So let's play family just for tonight.
Once all Delia's hip friends want to see ghosts, she needs her stepdaughter's help to impress her friends. But Delia can't stay this self-absorbed forever, right?
By the end of the movie, she's been trapped inside her own artwork and forced to watch as her stepdaughter nearly marries a perverted ghoul. An experience like that gives you perspective.
And then Delia finally chills out. She agrees to share the house with the Maitlands and let them undo her makeover. No more crazy postmodern interior design. She even manages to get herself on the cover of Art in America and, judging by her Betelgeuse sculpture, she finds some inspiration in unlikely places.
One last note: Catherine O'Hara had to be personally convinced by Tim Burton into taking the role. Once she heard that Michael Keaton would play Betelgeuse, some of her fears were allayed:
Basically, I just thought of myself on my worst day. Initially I thought she was going to be this one-dimensional witch. I mean, she is the villain and everything, but instead of making her just mean, which is how I first read it, she just became someone that gets in everybody's way because she's so insecurely self-obsessed. (Source)
Almost makes you feel sorry for Delia.