First of all, we dont even know if Rob and Kristen are a couple or not.
Second of all I find your question a bit rude. Who is to say who is a better person or who deserves someone else? What I mean is, we dont know Rob and Kristen personally, but they both seem to me like good people. You cant say you think someone can "do better", just because we like Rob. Maybe Kristen could do better than him! If two people decide to be a couple they deserve each other, and no one else's opinions matter.
How would you like it if you were dating someone and people went round saying how your new partner could get someone "better"? For all we know Kristen might be the sweetest kindest person ever, maybe Rob is super lucky to have her.