Why Somerville's force damaged Mers-el-Kebir so quickly

Did Somerville's force damage Mers-el-Kebir quickly because of good spotting or close visual range?

Both. The force had effective spotters and was close enough to sight its targets clearly.

The rapid damage at Mers-el-Kebir came from a combination of range and fire control. Somerville had good spotters supporting the bombardment. His ships were also close enough to obtain clear visual sight of their targets, so accurate spotting and short engagement distance reinforced one another.