There are enemies to use for a low-level Underdark setting. Not every cave in the Underdark is filled drow Matron mothers and high-level monsters...often the upper tunnels that lead to the surface house mostly low-level cave-dwellers but there are plenty of creatures to keep it interesting.
From:
https://www.reddit.com/r/DnD/comments/6hswjz/dming_monsters_of_the_underdark/Myconid CR 0,
Drow CR 1/4, Grimlock CR 1/4, Kuo-Tao CR 1/4, Vegepygmy CR 1/4,
Chitine CR 1/2, Deep Gnome (Svirfneblin) CR 1/2, Myconid Adult CR 1/2, Piercer CR 1/2, Rust Monster CR 1/2, Gazer (Beholder) CR 1/2,
Duergar CR 1, Kuo-toa Whip CR 1, Quaggoth Spore servant CR 1, Thorny CR 1,
Ettercap CR 2, Grick CR 2, Quaggoth CR 2, Vegepygmy Chief CR 2,
Hook Horror CR 3, Cave Fisher CR 3, Choldrith CR 3, Spectator CR 3
