Hello Tiffany! Welcome to the Disney Parks Moms Panel.
When feasting on
Minnie's Beach Bash Breakfast at Cape May Cafe in Disney's Beach Club Resort, the theme is always going to be "Beach Fun".
For the times that I have been at
Disney's Beach Club Resort or had breakfast at Cape May Cafe, the characters are wearing the "beach best" outfits to fit in with the ambience and enjoy the luxury of the
Stormalong Bay Pool Area.
Unless a overhaul or change is made to the venue or the Character Dining experience, I have not seen characters change outfits at a specific dining locations based on the time of year, with one major exception.
At
Hollywood & Vine in Disney's Hollywood Studios Theme Park, Minnie's Seasonal Dining is an experience exactly like you mentioned in your inquiry. Minnie Mouse and a few of her pals wear different outfits depending on the Season they are currently celebrating. It's a great opportunity to bring a little seasonal fun into a cherished experience.
Starting September 4th, 2017, Minnie's Seasonal Dining will also be a Character Dining Experience at Hollywood & Vine for lunch as well. Expect a few Halloween-themed surprises if you plan on visiting around that time.
Bon Appétit!
~Todd