Is Pebble Slime worth farming?
Only at the very start. You should replace it quickly once better route options appear.

The baseline Plains slime. Useful mainly as a teaching reference and lowest-tier progression benchmark.
Treat Pebble Slime as a baseline reference for how far your route quality has evolved.
Its role rarely changes because it mainly exists to anchor the very bottom of the progression ladder.
Only at the very start. You should replace it quickly once better route options appear.