Has there ever been another pop star quite as shameless as Ashlee Simpson? Probably so, but nobody has ever quite so cravenly followed fashion's shifting tides as Ashlee, who has never seemed the slightest bit embarrassed to make herself over when styles changed.

All this desperate trend-chasing has been done in public, as it damn well should be in the 21st century, so we've seen her change from the spunky younger sister of a superstar to the sad goth clown of her sophomore effort to the Gwen Stefani wannabe of her third album, Bittersweet World.

Ashlee might look like a shadow of her former self on the album cover -- the years and cosmetic surgery have made her virtually unrecognizable from the awkward teen on the cover of I Am Me -- but she still sounds the same, still boasting that same thin, girlish voice that wouldn't have gotten much attention if she weren't Jessica Simpson's younger sister.

Of course, the ironic thing about Ashlee's career is that she not only had bigger hits than Jessica, she made better records than her sister, too, all with a virtually nonexistent voice and a personality as aggressively shallow as Avril Lavigne.

Like Avril, Ashlee has a distinct arc to her three-act career, bouncing back from a dour and dumb second album with a return to the fizzy fun of her first (unlike Avril, Simpson seems like she would at least wait for you to leave the room before she started saying mean things about you).

Where Avril beat a retreat to the bratty punk-pop that brought her fame, Ashlee has pulled a red hoodie over her head, amped up the dance beats, revved up the '80s retro fetish, and created something that feels of the 2008 moment, as it should coming from the fiancée of Fall Out Boy's Pete Wentz.

This embrace of MTV hipsterism -- never to be confused with underground movements, this includes cameos from a guy from Plain White T's -- helps mirror the growth of her initial fans, who have grown from high school through college to immature young adults, needing this absurd new millennial go-go music for their endless parties, and while that arc is as manufactured as anything else surrounding the Simpson empire, there's none of the sad, creepy abandon of Britney Spears that makes Blackout just no fun to listen to, no matter how good it sounds.

Bittersweet World is all bright neon colors and bubblegum melodies, full of naggingly insistent hooks and insipid poses, none sillier than Ashlee boasting she's a "Rule Breaker" who loves to fight over a track that sounds like diluted M.I.A. When Bittersweet World is operating at this high-energy level -- copping from bad old new wave singles ("Outta My Head [Ay Ya Ya]") and Prince (the delirious "Boys") and Gwen (pretty much everything else, but especially on the feigned social consciousness of the title track, "What I've Become," and the "Hella Good" rewrite "Hot Stuff") -- this is addictive pure pop trash that's all the more irresistible because it's delivered by such a purely trashy pop star.

When things slow down -- as they do on the utterly forgettable closer "Never Dream Alone" and the quite awful "Little Miss Obsessive," where Ashlee explores the endless possibilities of the word "over" in the chorus -- it's a bit of a slog, but those moments are fortunately few and far between here because Ashlee is aggressively shallow.

She's always been this way, of course, but Bittersweet World is the first time that she has made a record that lives up to her happily empty persona, something that's truly fun junk.