Do i need to protect a DC adapter from current spikes (produced by buck converter)?
C2 helps with efficiency, else you're relying on the output capacitor in the DC adapter to deal with the ripple current, which might exceed its capability.

it depends on lots of stuff, a linear adapter may not hande ripple wall 
at all, but a switcher will likely handle it better, but nay not be able to maintain full rated output.

plus you also have radiated interference from the DC cable, especially if it's a flat pair.

better to spend 10c on an input filter capacitor