Gerations ago, The Giants once protected the Crystal Ponies, but were frozen by Sombra. They might look like statues from the outside when approachng the gate, but if the Crystal Princess or a Skylander arrives, they are alive on the inside. Now the Crystal Ponies have to reunite with the Giants, in order to stop Sombra.
I really like their style change but it seemed like a total waste to do it considering they only showed their style for like 5 seconds before it went to the next scene where they had left the crystal heart's range of magic thus turning them back to normal.
They probably couldn't think of a way to 'crystalise' the hat without making it look strange so they didn't care to try. I also think that there was a tiny bit of mind magic there, making them think 'Crystal Pony' and adopt a little of their aesthetics whilst they were inside the city's limits.