Sorry it took a few days to reply. . .
No, there is no problem taking food back to your room.
With the way the buffet is set up at Topsiders, you grab a tray, fill it up and then take it to a table either inside or outside. However, it is just as easy to keep walking past the tables and head back to your room. (Topsiders is much more convienent for this than the buffets in Parrot Cay, since Topsiders is more self-service than Parrot Cay)
You can pretty much take any food you want back to your room (or to almost anywhere on the ship). On the 7-day during the Midnight Dessert Buffet, they have silverware/cloth napkin sets so you can take your plate anywhere you want. If you are too full for dessert some night after dinner, but may want it later, just ask your server and you can take it "to go".