I know opinions are like posteriors, but for my part:
I absolutely love the artwork here, but I feel that the shirt would do just as well (or better) without the slogan. Simply wearing a "Unicorn Steakhouse" shirt is wonderfully ironic on its own.
Perhaps the slogan was added as an attempt to draw in more male customers, but personally, as a guy, I'd wear this without any pandering. Heck yeah, I'm secure enough to wear a unicorn on my chest!