The only scene that shows Fond Feather’s cutie mark. Unfortunately at a distance and not great resolution, and I still don’t know what it is, but it’s a thing that’s whitish on one end and blue along the rest of it. Possibly a blue cupcake with white icing? Or a cup of shaved ice? A cup of cappuccino? A vase of light-colored flowers? A cloud over rain? (Yes, I know that’s a pegasus stereotype, but it’s a stereotype because it often fits.) Something else?
 
The other two Bimbettes have odd food items (garden vegetables, and ketchup or jelly on a baguette) so I lean towards a frosted blue cupcake or a blue mug of hot cocoa with whipped cream on top, to stay in theme with them.