Difference Between RMS noise and PSRR in an LDO
The noise is internal to the LDO and is always on the output whether you power it directly from an SMPS, a battery, or the most perfect noiseless voltage source.

The PSRR is how well the LDO can stop ripple injected into its input from passing straight through to its output. Really, it's how fast the control loop inside the LDO can react to adjust its output to account for changes on the input.

So in your case, you would expect the output to have the SMPS ripple supressed by 76dB with the noise on top of it.

If you powered the LDO directly from a battery, you would only see the noise on the output. No ripple.