Is it true that a tumor on the optic nerve is dangerous?

Possibly. Tumors anywhere can be dangerous. Optic nerve tumors are most commonly gliomas which are not malignant but which interweave with the nerve, disturbing vision and removal is frequently accompanied by permanent loss of vision. This is not "dangerous" but loss of vision is not a good thing. Other optic nerve tumors are more rare but do need to be inbestigated.