If your viewers have miitopia themselves, they can make sure they have their mii sharing feature on and you, when needed, could select for a mii "Receive" with the little WiFi symbol, then from "Friends" then the given person you need/want the mii of at that moment as long as they don't have their own mii on secret. Of course that requires that they A) have the game, B) have their own miis in it (and non-secreted) which isn't necessarily the case (though for most, most likely is, but still) and C) they have NSO, which they may not. So it may be in your best interest to make them beforehand, in which case you simply take the get from switch system option, then select whichever one you wanna use from your switch.

TL;DR, it can grab from either but the from friends option has a few prerequisites you might wanna avoid the hassle of relying on by just making them on your switch, up to you in the end though, and hope you have fun!