I went to Universal last year and visited that same shop, and bought several T-shirts, and your right... this was noticed when my stepmother was looking around.
I will point out though that a lot of the other things for sale like messenger bags, coffee mugs, graphic novels, pins, etc... are all pretty equal oppertunity. Picking on the T-shirt selection, even if that WAS a good portion of their inventory, seems a little petty.
That said, to be entirely fair, whether it's at the casino where I worked, or other places I've visited, girls don't generally seem to be a big market for T-shirts to begin with. I think this has a lot to do with the differance in selection outside of a department store, just about everywhere has less of them, and for that matter I see less girls browsing T-shirts to even notice. In the time you were there how many other girls came in and made note of the same thing, or even just browsed them with interest trying to find one like you did? I'm sure the people who work the store have seen plenty of people do it overall, but in proportion to those that visit it?
It might help I think if more girls were interested in the whole souveneir T-shirt thing, at least as far as wearing them goes (as opposed to putting a band T-shirt on your wall or whatever). I understand the arguement that "well maybe if there were more of them, more girls would be interested" but really when you wind up with businesses already over-producing shirts and needing to dispose of them or ship them off to discount joints, it's understandable that faith is in short supply. Really, I think increased demand needs to exist.
See, things would be differant if there were multiple ladies coming in every hour of every day and complaining about the T-shirts in that marvel store. I'd imagine they MIGHT get one or two a day, if that.
As far as the "insulting" blazons, well that seems pretty typical. It's not insulting so much as ironic to be honest. Like it or not you'll find variations on that just about everywhere in a number of themes, because that's what sells. Generally speaking shirts like that are worn to goof off when your out with someone more than anything, and yes it DOES say something when that's all you see... but that something is that they are being as generic as possible since they figure they are more likely to find someone interested in wearing a shirt like that for the lulz for a day of fun, and in greater numbers, than a geek girl who is also interested in buying and wearing a T-shirt. Those shirts are in the same basic spirit as say parents buying the "Thing 1, Thing 2, etc..." shirts for their kids for the day from the Doctor Seuss island... which is probably even more disturbing if you want to try and read into it literally.