Say what you will about Ashlee Simpson’s hair (back to her natural blond), nose (surgically smoothed, supposedly) and boobs (apparently quite real, and not to be featured in Playboy despite a $4-million offer), the brat-pop princess’ defining characteristic may well be her thick skin.

How else could Jessica’s little sister continue to record and tour under the withering weight of the jokes, parodies and MySpace slaggings she has endured over the past couple of years? From her drunken escapades at a Toronto McDonald’s to the infamous Saturday Night Live lip-synching debacle, this girl has turned being mocked into a career-defining hallmark.

So kudos to her hair, nose, boobs and her tough yet shiny-soft skin. Now, there’s the small matter of that voice …

Uneven would be the kindest way to describe Simpson’s less-than-half-sold-out show at the Molson Amphitheatre last night. After openers Chris August and adorably squeaky clean former O-Town frontboy Ashley Parker Angel, the 21-year-old Texan got out there and did what Ashlee Simpson does best. For better or for worse.

Backed by a tight band (including Simpson’s guitarist-boyfriend Braxton Olita) and a pair of backup singers (no “guide track” in effect, as far as we could tell), she launched straight into Boyfriend and was visibly shining with sweat by the time she slid into Burning Up.

She was nothing if not enthusiastic, shaking her butt — which was later amply revealed by the fishnet stockings that made up one of her many outfits — from one end of the wrought iron-accented stage to the other.

The odd cover was mixed in during the 90-minute rollout of her peppy but generic repertoire, including a so-so take on the Police classic Message In a Bottle, a bizarre mangling of Peggy Lee’s Why Don’t You Do Right? and a surprisingly raspy-good rendition of the Eurythmics’ Sweet Dreams. Though the white gown and black top hat she wore for that one screamed Stevie Nicks, not Annie Lennox. Or Marilyn Manson for that matter.

Given the lip-synching fiasco, it was a little ironic — or maybe completely deliberate — that her best songs were her stripped-down acoustic numbers, including Beautifully Broken and particularly Undiscovered, which sounded almost like a new-country ballad. In a good way.

It’s when Simpson really belts it out that things get messy. She can strut and sweat and pass on earnest teen-friendly messages about not letting negative people get to you, but she just doesn’t have the pipes to put her in the same league as contemporaries Avril Lavigne or even (eek!) Hilary Duff.

Maybe that’s why the audience of 3,000 or so mostly mid-teen girls didn’t really seem to get into the show until the final 20 minutes. Sure, they screamed on cue when Simpson introduced one of her better-known songs, or talked about her reality show, or mentioned Canada in any way. But they weren’t off their butts and moving en masse until La La, L.O.V.E. and Pieces Of Me closed out the night.

But if Simpson noticed the lack of unbridled energy from the small-ish crowd, it didn’t seem to faze her a bit. Which isn’t really surprising. The girl has some pretty thick skin.