I used the nothic in phendelver. My players were intrigued and terrified with a green flavoured mind monster.
If you had a city of them, I'd imagine you'd have this cacophony of mental discussions. Kind of like being in a crowd, but noone being around. This could create some healthy speculation amongst the group. Maybe you could make some of the players more sensitive to the talk giving them lore, but a disadvantage if it gets too loud.
Darkness was the main pull of the lair in phendelver, so I'd lean on that. Lots of hiding spots and mind speak makes the monsters seem hard to find which adds tension.
Let us know how you do.