Hello, Jeffrey, and thank you for your question!
Fantasmic! is a classic
Disneyland Resort show full of water effects, pyrotechnics, and appearances from some beloved Disney characters. If you would like to bring to the resort a cushion so that you can enjoy the show more comfortably, you may do so. In fact, I often like to bring a small blanket with me when I plan to watch Fantasmic! for a similar reason.
However, walking around the Disneyland Resort all day and all night with a cushion could be tricky. One recommendation I have for you is to consider
renting a locker. You can place the cushion inside and then take it out when ready to make your way toward the Rivers of America for the show.
Enjoy - you are in for a once-in-a-lifetime experience!
Best wishes,
Lola