Sorry if this is covered already but I couldn't find solution...
Anyway:
Is there a way to display the hostname of my PiHole instead of 'localhost' on client stats page?
Tried modifying hosts without any change.
Or even better (IMHO) hostname with highlighted or bold letters to clearly show what it is.
You need to edit /etc/hosts and add 127.0.0.1 theNameYouWant
A system reboot is required for the settings to be reflected in the /admin/ interface.
Ohh, it was that easy! 
My bad!
Thanks!!
It could still be an idea to mark the 'localhost' (real name) with bold, another colour or something else so it differs from "normal clients" if You see what I mean.
Edit:
I.e. like this
- pihole (localhost)
Why do you want to differentiate localhost from the other clients?
Just to easier visually differ it from other 'clients' as I see my Pi-Hole as a 'server' (part of infrastructure services) and shouldn't normally have that much outgoing traffic except for updates.
You can also use the IGNORE_LOCALHOST FTL config option if you want: Configuration - Pi-hole documentation