Fonts were released in Season 3: Warp Speed. 

When a user selects a font, their player name will now be displayed with that font. This will be on the in-game leaderboard, on their profile, on the kill leaderboard, etc. 

Currently there are 7 fonts in Smash Karts (including the Default one).

Fonts are available for $ and when buying the Smash Pass. The last 3 fonts (Violet Visonary, Pretty Prism and Flaming Red) are not publicly available and are Rare Items.