Is feminism about defending women from criticism?

I certainly don't think so. The notion that it's sexist to criticize women is itself sexist. Address the criticism. If it's unfair, say why it's unfair. But let's read this argument from Mary C. Curtis:

Michelle Obama has become an issue in the presidential campaign even though she isn't running for anything.

She's going around promoting her husband and is therefore an important political actor who is part of the debate and subject to criticism.

An educated, successful lawyer, devoted wife and caring mother has been labeled "angry" and unpatriotic and snidely referred to as Barack Obama's "baby mama."

Democrats, Republicans, independents, everyone should be offended.

Offended? I'll push back when the criticisms are bad, but not because a woman has been criticized. When a male politician has done something wrong, we don't moderate our attacks with obsequious celebration of his level of education, his professional success, and his purported adoration of his wife and kids. And even when the attack is unfounded, we aren't offended that there are attacks. We fight the attack.

And this black woman is wondering: Where are Obama's feminist defenders?

The author informs us of her race. Why? Does that give her special moral authority?

It's not as though they're out of practice. In 1992, Hillary Clinton was deemed too assertive and not first lady material. Similar, and worse, claims were made this year. But just as you didn't have to be for Clinton to decry the sexism in the coverage of her campaign, you don't have to be an Obama supporter to defend Michelle Obama against similar treatment.

I certainly agree that anyone who got righteous about Hillary Clinton and sexism must apply the same standard for Michelle Obama. If you want us to believe you were standing on principle, don't be a hypocrite.

But I'm not one of those people, so I'll say what I've always said. Women in politics are subject to attacks, as are male politicians, and they aren't going to succeed by arguing that the attacks are sexist. Get to the point and tell us why the attacks are wrong.

I've long been frustrated, as a black woman....

In case you forgot, the author is black.

[I]n America, there's seldom a cost for disrespecting black women.

Seldom... she didn't say never. The fact is, Americans feel much freer to disrespect white men — and this is not a disadvantage for white men. This is part of being viewed as strong and powerful. So be strong and powerful, quit whining, and fight.