Basically- incest warning for the topic but...

This is really interesting as a fan lore question- Why is Button Mash nearly always intimate with his mother in fan art? Where did this start? I mean obviously it’s non-canon, so how did this begin in the fandom? Anyone know? Did any specific artist start the trend? Thanks!

Best suggestion I can make is search Button Mash on Derpibooru from the earliest date and see where it develops. Pretty sure the whole thing originated there (or maybe /mlp)

IIRC the joke was made immediately after the fan animation featuring the two characters was made, by the VAs. Earliest fanart representation of that I can think of (fan art of fan animations?) was from Tiarawhy.
Keep in mind that everything about "Button" is pure fanon; I only recall him as showing up exactly once in "The Perfect Stallion" in the show itself. Might have shown up more in the comics.