The Smash Bros. Fighter Ballot was a feature added to the official Smash Bros. site that allowed players to request characters for Super Smash Bros. for Nintendo 3DS and Wii U. This ballot started April 1st, 2015 to October 3rd, 2015.
In this site, players were presented with a small form where they add the character's name and game of origin, as well a reason to justify why they want that character. Players could enter their suggestions until October 3rd, 2015.
The results from each region of the globe were taken and compiled, with characters from previous games being put together with newcomers. The ballot also featured fighters that were already in the series. Nintendo considered the characters' results in each reason, as well as the realizability of the character (how easily Nintendo could get the rights to include them).
Though Bayonetta was chosen as a "winner" for the ballot, data mines in early June of 2018 suggests that Bayonetta was actually chosen very early into the ballot. The data mine includes information about the 1.0.6. patch update which contains her parameters. The 1.0.6. patch released April 15, 2015 which is nearly two weeks after the announcement of the fighter ballot. After these allegations that the Fighter Ballot was supposedly rigged, Sakurai had stated that the Fighter Ballot was not used for the next DLC character, but was instead chosen to determine the roster in the next Smash game (later revealed to be Super Smash Bros. Ultimate). This statement complies with most of Ultimate's roster as many of the characters that were voted highly in the ballot gained fighter access in said game (King K. Rool, Inklings, Simon Belmont, Ridley, and others).