The reasons most people tout around seem to be;
-She was given a fully evolved pokemon right at the last minute, only a mere handful of episodes before the Grand Festival
-She had never used Togekiss prior to being handed it
-Princess Salvia was considered a cheap, uninspired character by some as she was essentially a "prince and pauper" version of Dawn
-There was a huge amount of hype from certain circles for years that Togekiss was a surefire excuse to bring Misty back
-There was also much speculation that the evil Togepi would return, be captured by Dawn and evolve, or that Cynthia might be involved... basically the speculation between the revelation of Togekiss and the actual capture was all more fanservice-y than what we wound up with.
-Togekiss was used in a single major battle and lost, and what's more it was a battle they'd built up the entire saga for
-People were still sore over Ambipom's ridiculous departure
-Togekiss was used to weigh in on the running gag of Gible's Draco Meteor hitting Piplup, which was a base breaking subplot
-Togekiss's personality revolved around it being a prima donna
All that said, I personally really liked Togekiss and didn't see what all the fuss was about. I've even seen it named as one of the major downsides in DP, or a jump-the-shark moment for the franchise. I think that's downright silly. She was no more cheap than Ash's last-minute Glalie, and let's not forget how powerful Gible suddenly became at the eleventh hour. She was entertaining in her time on the show, gave us a great aerial battle and showed Dawn's growth as a coordinator. I miss her more than Buneary, Pachirisu or Quilava, and they don't get the same criticism.