I think there's a fundamental difference between momentary facial expressions on the one hand, and a conscious clothing decision on the other hand.
In a trial lasting many months, it's entirely unreasonable to expect the defendant to maintain a neutral, sober facial/body expression at all times, especially at the start or end of a day's proceedings or during a break. And it's therefore a virtual certainty that cameras will pick up the occasional smile, laugh, goofy face or whatever. And of course those are the pictures that are beloved of photo editors, so they tend to get undue prominence. It can sometimes therefore give people (especially people with an anti-Knox point of view) to draw the incorrect inference that Knox laughed and smirked her way through the trial. I don't think there was anything she could have done about that, and I also think that most reasonable people are able to understand that it's perfectly natural (and virtually inevitable) for there to have been times when Knox smiled and laughed.
But a conscious clothing decision is, in my eyes, a completely different kettle of fish. That's not a spontaneous or instantaneous happening. That's something that requires consideration, a certain amount of planning, and ample opportunity to rectify. And, as I said, her lawyer simply shouldn't have allowed her anywhere near the courtroom (or the cameras....) wearing that t-shirt. Assuming of course that he or the other lawyers actually saw her before the start of that day's proceedings, as they should have done.