That's not how sexism works. Pointing out obvious sexism is not sexist. Recognizing that something is obviously sexist is also not sexist. We are all well aware how certain (hell, most) toy and clothing brands market to children. This sort of uber-skepticism* is ironic because it lacks any ounce of real critical thought.
*Neo-skepticism, maybe? Heh.
(Side note: It's already been noted that similar anti-intellectual themes aimed at boys also exist. But that doesn't make this particularly shirt any less sexist, and there is plenty of context in our world, and plenty of history in marketing and consumerism, to see that.)