Hello S.,
You can use creole for that with <font> and '\n':
@startuml
if(some condition) then (yes)
:;
endif
if(some condition) then (\n<font:monospaced>yes</font>\n\n)
:;
endif
if(some condition) then (\n<font:serif>yes</font>\n\n)
:;
endif
@enduml
If that can help,
Regards,
Th.