Is critical strike multiplier a more or increased damage modifier?

No, it is not.

It is a multiplier, which is neither a more modifier nor an increase modifier, and does not function like them - there's a very big difference between a multiplier of 150% (1.5 x damage) and 150% more/increased damage (2.5x damage).

The critical strike multiplier is a multiplier on the total damage of the hit. The way it applies to damage is not changing at all in ascendancy (the stats you get to modify the critical strike multiplier are, but that doesn't change how the multiplier applies to damage).