Pi-hole is keeping resolving "dnsdumpster.com" is this expected?

I don't know why, it's seems to be coming from the Raspberry pi himself.

Is this coming from the youtube ads script : How do I block ads on YouTube? - #88 by Joshua_joezoef_B_ZzO
and is it safe?

It's not expected from a normal install of Pi-hole. You should contact the author of that script for more information about its behavior.

ok thanks I will forward to the correct post