PSRR of buck converter (or boost)
To answer your questions:

&quot;Is this ripple problematic for a DC/DC converter?&quot;

The ripple at the output is directly related to your output capacitor and your output inductor. If your ripple is too high that means the current through the inductor is too high or your output capacitor is too low.
In the case that your output cap is too low there's not issue from a converter perspective given that you're not exceeding the maximum stress voltage on your transistors (highly unlikely). However, just because there's not problem from a converter perspective doesn't mean that you have a solution. You'll have to figure out if your load is sensitive to this ripple or not.
In the case that your inductor current is too high (the current grows linearly at a rate of (Vin-Vout)/L * t) then your inductor might saturate (if magnetic) and straight up burn whether magnetic or air core.

&quot;Why Power supply rejection ration (PSRR) is never specified for DC/DC? Is this value meaningless?&quot;

The ripple at the output is a direct function of output inductor, output cap, and switching frequency. The control loop takes care of noise in your input supply (provided the noise is within the loop bandwidth). If it's higher you wouldn't see it.

&quot;Why people always use LDO after a line frequency bridge rectifier?:

It's not &quot;always&quot;, it's application specific. If your load can handle the output ripple you can get away with using a dc/dc converter. But in general applications the manufacturer doesn't know how it'll be used to they air on the side of caution and give a steady output.
Also, it may be cost related. dc/dc converters are always more expensive due to it's control and inductor. Many times more expensive than a simple LDO.