The strategy for Nintendo Switch Online is apparently to dripfeed classic games onto it. So if you for example want to play Mother (NES) or Mother 2 (SNES), you either need to emulate it on a PC, buy the actual retro consoles and games (or the SNES Classic for Mother 2), or get a Wii U and play it on the Wii U Virtual Console.

(If you want to play Mother 3 in English, you're going to have to resort to PC emulators.)