The best prank text shortcuts are those that the other person uses regularly. For instance, changing lol or lmao or hi to something completely unrelated will definitely cause confusion. This works best on those who aren’t particularly tech-savvy and have no idea how to operate their phones other than to send messages and share memes.
This person changed added a shortcut in their mom’s phone to say “Twinkies” whenever she wrote lol. You can sense the confusion from the mother just by reading her following text messages. If you want to take the prank to the next step, say that her phone is “broken” and that you can get it fixed for $50.