Hello A.,
You can use those workarounds:
1. Globally change the defaultFontName for all text
skinparam defaultFontName monospaced
state "State Name" as S1
See also: https://plantuml.com/font
2. Locally change font for specific text
state "<font:courier>State Name with Text on courier font" as S1
state "State Name 2 with <font:monospaced>Text on monospaced font" as S2
[See on PlantUML server]
If that can help,
Regards,
Th.