I've been using the iOS application for 2 days now and still no hat.
Is it some sort of bug?
As a matter of fact, I'm writing this question on my phone.
Oh... Well, the text
use the iOS application isn't very descriptive :|
And there it is. You have your hat.
Using the app is not enough. According to the description:
post, vote, edit, or comment with the iOS app
I guess you did now, so there you have your hat.