@love_hippo - There are currently only two fonts for Sticky Notes.

There are many Wish List Idea posts requesting additional features to Sticky Notes - I would suggest adding your comments/votes to any of interest:

https://community.miro.com/search?q=sticky%20note%20font&content_type%5B0%5D=ideation

The one does mention adding “more font options”
https://community.miro.com/wish-list-32/free-text-formatting-for-stickies-font-colour-250

Some relevant Help Center articles

• Sticky Notes (no mention of font types)
• Fonts

  

Workaround

One hack would be to use a rectangle Shape and make it the same colour as a sticky note.

Nerd alert: Attached are the Sticky Note HTML color codes.

I suppose that you could also use the switch type action on a sticky note to end up with a colour-matched shape.