I've been having a debate with someone who says that Super Mario Run isn't a main game. I want to know if it is technically considered a main game or a side game. Is it a main game or is it a side game?
Personally I'd consider it a spin-off, but iirc Nintendo themselves have stated that it's a main series title. I can't really be baffled at that since Super Mario Bros. 2 is also a main series game despite all the differences between it and other Mario games.
Nintendo states that Super Mario Run and Super Mario Maker are in the list alongside the other main games. I would also consider New Super Luigi U and Captain Toad: Treasure Tracker (and maybe Super Mario Bros. Special) as side games, because they are related to the main games.
I feel that Super Mario Run is part of the main series as, not only does it bear the "Super Mario" name and is a platformer like the original Super Mario Bros., it also is being worked on by the developers by the original Mario games. It might not have everything in common with the traditional game, but I think that the fact that Nintendo treated Super Mario Run as a big event (remember 2016's iPhone reveal, where Miyamoto shows up live?) meant that they treated it as a big game.
It's not really a main series title when it's not a 2D platformer like the other games are. It's in its own genre of auto-runners, like how rail shooters are considered a distinct genre from FPS's.