Is C0G a synonym for NP0?
Yes they are equivalent.
C0G is the material class that gives an NP0 (NP-zero ) tempco.  But it is slang (NP-oh) where the 0 looks like O.
Beware that it is often spelt both ways with oh or zero, but it is intended to be a zero in both cases.
They also have P100 for +100 ppm/‘C and N100, N200 etc for -PPM over a greater range since ceramics tend to be offset this way.  The ceramic composition is varied for analog applications requiring this .
C0G: Normally used for low C values. It has a low dielectric constant, but highest level of stability vs T and Vdc
X7R: Used for high C values with much higher dielectric constant than COG, but a lower stability.
Z5U: Used for even higher C values but has a lower stability than either COG or X7R, thus lower costs

The other tempco ratings that are below NP0 have different alpha-numeric codes. eg. P2H R2H S2H etc
see Murata specs