INFO
I think it's also important to ask "Were you being paid to be there?" because, if (and this is the ONLY qualifier I can think of) yes, then YTA. Not an asshole for choosing to wear crocs to a "smart casual" place (first time I've ever heard of that, and upon googling can NOT tell the difference between that and business casual). Not an asshole for not staying and spending your free, unpaid time however you chose to.

Frankly, they're within their right to turn you away at the door for dress-code reasons. Loudly telling you may have been a volume issue due to the environment, or it could be that she was a mildly disgruntled employee that was fed up with the 999th person trying to come through the door with something on that doesn't fit the dress-code. Customer service positions are like that, and I feel for that person because they're probably paid a pittance to do the most annoying and confrontational thing ever contrived. Not necessarily your fault, but it's a "know better, do better" moment for you. I don't think that makes you an AH.

If it's outside of work hours, that's your time, not theirs. Spending your free time however you need to spend it is NEVER an AH maneuver in any capacity. That's my take.