Once, I had a professor who pointed out that there were studies done on how much better people do on presentations if they dress up... What we wear affects us, consciously or subconsciously. That's that...
So.. if you wear things like:
"Can't listen, you are dumb"
"It's all about me, deal with it"
I'll think of you a certain way... I'll make certain judgments about parents who dress up their kids in certain things (and I'm not talking about princesses or even Disney characters here, but things that make vulgar statements). I think as a parent, you have to make sure your daughter understands that many will think of her as a selfish, self-centered being if she walks out of the house like that.
Why? Because those are selfish and self-centered statements, and obviously, she endorces them by wearing them.
Just because it is funny to you, doesn't mean it's not mean-spirited.
Humor doesn't have to be mean-spirited.
These are very often (not always, but often) the same people who want messages on clothing to express their identity...or to give them one.
Those people want an impossible thing: they want clothing to define (or create) their identities but don't want to be classified or labeled as belonging to those identities. They want to mean something and not mean it; they want to express an idea and not be subject to the consequences of expressing that idea.
The reality of the matter is just as you said: people do judge you by what you wear...and rightly so. QUITE OBVIOUSLY, the sum total of a person's identity cannot be determined by one t-shirt and there may be a host of reasons why that person is wearing a t-shirt with a mean-spirited message on it, but for the most part, one's clothing -- including a t-shirt with a message on it -- provides a significant clue about how a person wants others to perceive her or him. Every woman who covers for modesty knows that, to take one example.
All that I can say is not to be surprised when people use their judgment -- the ability to come to a conclusion based on evidence and one of the most superior qualities that human beings possess -- to come to a conclusion about the wearer, feeble protests of "Don't judge me!" to the contrary.