This handy guide to images will help you take the best screenshots on all the various platforms where Miitopia can be played!
Original game (2016)
On an actual 3DS
If you have not already read the game's manual, take pictures by "holding down Y Button, and pressing Up (Top Screen) or Down (Bottom Screen) on the D-Pad," anytime, anywhere!
With Citra Emulator
Please try to take screenshots (not the in-game kind) if you can, as the quality of emulated screenshots far exceeds those produced from the regular screenshot function.
Switch version (2021)
Use the console's snapshot button (located under the directional buttons) to take a screenshot. Hold down said button to record the last few seconds of gameplay footage (up to 30 seconds). The Y button can still be used to freeze the current scene, in case one needs to take a screenshot of something that is too easy to miss by the time the snapshot button is pressed.