Darlene's vegetarian "principles" are a joke. She acts like she's the only one with scruples in the family. And not to mention the way she treats David; she treats David like a bag of garbage and yet she won't eat a hamburger. There's a discrepancy there. Maybe she should've gone to a therapist the way Jackie did. A true hypocrite.
A vegetarian asking for a leather jacket was kind of hypocritical, too. But my take on the whole Darlene-Holier-Than-All attitude is that it was actually realistic and well played. Hypocritical, annoying, obnoxious and pious know it all- absolutely. But pretty typical of a teenager who is trying to find an identity separate from her parents. Some kids that age are just convinced that they know better than their parents, they're smarter than their parents, etc.
Yet they don't see their own faults. As you said, she treated David like crap while acting like she's the only person in the family with principles. She also seemed to think she was so independent, yet her hand was constantly out for money. David was right to point out that her principles were somewhat plastic in that regard. She complained about the source of the family income, yet was eager to help herself to it.
Again, pretty typical teenager behavior and mindset.
Her real problem wasn't being a vegetarian or a teenager. It goes deeper. Her problem was her negative behavior was rewarded at home. Not only that she saw her mom treat others like garbage and not get in trouble/get rewarded. I'm a teacher and I see this alllllll the time. The only difference is irl, Darlene would drive her parents crazy. Roseanne thought she was cute. Of course, because she was Roseanne Jr. I think it's summed up perfectly when Darlene's teacher is disturbed by her barking and Roseanne retorts "We all bark". Um, no. The good daughter didn't bark.
In the opening sequence of the show where the whole family is seated around the table, it's plain that they're eating a pepperoni pizza. So, Darlene the vegan is eating it too.