Personally I think it is best just to move on unless they are asking for feedback on cover/blurb/etc. If it is someone you know in real life (or even through Facebook or other social media) you could send them some feedback about it too but I would make sure you have good constructive criticism for the cover, not just that it sucks.
On the passive-aggressive side, I suppose you could always submit it to
Lousy Book Covers* and if they accept it send that to the author so it isn't just one person's opinion. Or for print books
Good Show Sir works as well.
After looking at Lousy Book Covers, I would give the specific book in this thread a pass. It isn't nearly as bad as others.
*After reviewing GrannyGrump's post this appears to be the same content as the Tumblr site she linked.