Sorry Kirby haters, according to geotagged Twitter data from December 7th (Super Smash Bros Ultimate release date) to now, the little pink ball person is the most popular character... at least on Twitter.
A query was run in every state regarding the game (using Smash Bros hashtags and phrases) and the character with the most mentions was revealed in the results. Kirby had the most by far, and when the results were mapped out, he won over 20 states. No other character comes close.
While Kirby is one of Nintendo's most beloved characters, a solid explanation for him being talked about so much is that Kirby is pretty much the main protagonist in story mode, so the game somewhat centers around the character.
Even more interesting is that several top gamers have Kirby ranked relatively low on their tier grades (but again, most people pick a character simply because they like them).
The next most mentioned characters are Bowser and Pokemon Trainer (let's be honest it's Ash) each winning 5 states each.
So while this map might not show each state's "main" (the character a person most often plays with). It does reveal the characters people love talking about.