It's still hipster for a guy who's supposed to be all about military discipline and leading by example. I mean, it's not that anyone else would care, it's just out of character for him to neglect military regulations like that when it comes to his own appearance.

I still hold that Blood Line had the best portrayal of Mendez to date, not just because he looks appropriately tough but also because I've always imagined him as sporting a mustache (even though he's never described as such in the books AFAIK).