Well, yes, I think she is, in general, happy with her life: Anna know about her power and she's okay with it, her whole KINGDOM is apparently okay with all the Ice stuff, she can finally control that power - be herself - but I don't think happiness is something that you can just switch on and forget years of fear and problems.The sad things come back when you don't expect them, and, since the change in Elsa is both physical and psychological, I think watching herself in the mirror can start the thought of her past.And this isn't a normal mirror, it's (sorry for my bad english, I'm not sure I can make it very understandable) her crown. Something like "How I see myself as the queen that I was and I'm now. How they see me?" Because when her people (okay Hans wasn't an innocent prince and the duke of Weselton had already was already messed up the things, but still) come for her armed in the Ice palace she was already the queen. Or when she snap ad the coronation party.So yep, I think was a good idea portrait Elsa unhappy in front of a mirror with the sentence "I'm happy". I want that contrast.Thanks for the comment!