Portable power supply for over 300 ws2812 LEDs (similar to youtube video)
Your calculations are reasonable for every single LED on at full brightness. However, there are a few reasons why the LEDs don't draw this maximum power. So, if you want this dress to work for an hour at max brightness, you need 12Amp hours of energy. That's on the order of magnitude of a small car battery (not as much, but it's a big battery). Also, the voltage plays an important role but let's assume for simplicity's sake that she has a battery that's just the right voltage for her application.... So, why does she only use a "small" battery (for what it's worth, that's a pretty large battery compared to something in your phone)? :


Not every LED is on all the time, especially when she is doing blinking patterns. Let's assume that only 50% are on, on average. That brings you down to 6 amp hours. This drone battery gets you there. http://www.maxamps.com/proddetail.php?prod=Lipo-5450-111-Aton-Pack
Not every LED is on at full color. That 60 mA number you gave is all 3 colors on at once. So, if a light is full red, green, or blue, you're only at 20 mA. So, let's assume that only 2/3 color is on at a time, on average. So, you're down to 4 amp hours.
Not every LED is on at full brightness. The PWM signal that controls the LEDs allows them to only be on for a limited duty cycle to adjust brightness. I've used those WS2812s and they can get pretty darn bright. So, let's assume they have a max brightness of 75% current (especially in a dark room). That brings you down to 3 amp hours.


So, 3 amp hours is actually something that a normal spare phone battery can do, or many RC/drone batteries for higher voltage.