I think there's a difference between disrespect for the authorities, though, and disrespect for the dead / for veterans. I suspect she intended to just disrespect authority (i.e. whoever put in that sign and is trying to dictate that everyone must be respectful) and it didn't even occur to her that someone might interpret her actions as disrespectful to the deceased, the armed forces, or anyone else. I can especially see that if she was going there with a mindset of the middle ground - not thinking soldiers are particularly worthy of extra respect but not thinking their careers make them worthy of extra disrespect either. Some people see serving in the armed forces as a special thing and some people see it as a career choice just like being a doctor or a plumber.
That's a fair point as far as it goes, but this isn't just a case of soldiers deserving respect, let alone more respect than other professions. Reasonable people can disagree on whether soldiers are extra-worthy or not. This, however, was a case of disrespect at a monument dedicated to people who had died in the service of their country. I'm confident there'd still be more than a bit of outrage if she'd done this at a monument to dead firefighters, or cops, or doctors, or plumbers. Maybe the latter two professions wouldn't get as much outrage as the others, and that can make for an interesting discussion, but respect for the dead - all dead - is a pretty big deal in our culture, and I think it would've caused this to blow up even in other circumstances.
The specific circumstances are basically a perfect storm, of course. You decide you want to be disrespectful to soldiers, but that's not enough, so you move up to disrespectful to dead soldiers. But most of the graves at Arlington are just "regular" dead soldiers, who at least can be honored by their families and under their known names, so even that's not disrespectful enough. So you decide to be disrespectful to the dead at the Tomb of the Unknowns, which is not only an iconic and beloved American monument, but is dedicated to the untold thousands of people who have died for their country in complete anonymity. I can't even imagine how many people died in the Revolutionary War or the Civil War who were never properly accounted for, let alone other American conflicts. Those people will never get the recognition they deserve - literally never - but we built a monument to them and we guard it twenty-four hours a day and it has become one of the most sacred sites in all of America because it assures everyone who serves our country that we will honor you and remember you even if we have no idea you ever existed at all. It's a pledge from the American people that even the forgotten will always be remembered. And that's what this idiot deliberately chose to disrespect.
Now, all that said, I think your guess that she intended to disrespect the sign more than anything else is probably spot-on, but it just doesn't matter. Maybe she was intending disrespect to nothing but words on a plaque, but the concept that words and symbols have a deeper meaning than just their physical manifestation is pretty central to what it means to be human. Also central to our basic humanity is our desire to honor those who deserve it. She ignored the first on her way to spitting on the second, and if the best defense anyone can come up with is "She's really just that stupid," I can't particularly motivate myself to care.