I have a Windows machine at work which runs MS Office. Excel recently got an update which sets the formula bar in a monospaced font. Imho, it is awesome. I really do like it.
Is it possible to format the font in Calc’s formula bar in a user-defined monospace font?
No. You could make an enhancement request, How to Report Bugs in LibreOffice - The Document Foundation Wiki
Bug 162238 - UI: Monospace font in the formula bar?
duplicate of
Bug 95406 - UI/Editing Feature Request: Font in Formula Bar
Lol ok? That was uber fast. 
Hm, shame. 