We can gain a better understanding of Claudio's thoughts and feelings the moment Hero unveils herself in the final scene by first looking at what we know of his thoughts and feelings prior to this scene.
We know that one thing Claudio was feeling at the moment just before the unveiling was a deep sense of grief and repentance. We begin to see Claudio's repentance portrayed the moment Borachio confesses that, by Don John's urging, he had tricked Claudio into believing that Hero was Margaret and that she was unfaithful. Claudio calls Borachio's confession "poison," referring to how his confession has just killed his soul like poison, as we see in his line, "I have drunk poison whiles he uttered it" (V.i.234). Claudio also exclaims "Sweet Hero" and proclaims that now he is seeing her as the pure woman he fell in love with, which he regrets because it is too late, as we see in his lines, "[N]ow thy image doth appear in the rare semblance that I loved it first" (239-240). He also begs Leonato to revenge himself for Hero's death by any means Leonato sees fit (261-263). Not only that, Claudio writes an epitaph to hang on Hero's tomb proclaiming her innocence and her wrongful death, and he begs forgiveness of the goddess Diana, a goddess known for her virginity (V.iii.3-11, 12-13).
Since we see how deeply grieved Claudio has become over Hero's death and how deeply repentant he has become, we know that when who is actually Hero is unveiled, he will feel a profound sense of joy and relief. Also, we know from Claudio's line after Hero is unveiled, "Another Hero!" that he feels the same thing Hero does. He feels that the Hero he unjustly slandered and violated has been reborn anew, fresh, and undefiled. Hero expresses this sentiment best when she says, "One Hero died defiled; but I do live, / And surely as I live, I am a maid" (V.iv.65-66).
Hence, we know that due to his profound sense of grief and repentance, when Hero is unveiled before Claudio, he feels a profound sense of joy and relief. We also know he feels that she has been born anew, undefiled, and pure, just as Hero declares herself to be.
Being Claudio at that very moment I would be very shocked and syunned as it would be a very unexpected to literally see hero reserected. However, Besides from the ultimate shock i would settle into relief and happiness for many reasons. Firstly i would not have to live with the guilt of killing Hero. Secondly i know that i would be maarying a woman that i have always loved from the beggining of the play which would make it a lot more easier for me to marry her. Nevertheless i would still have a strong feeling of regret and shame as I know that it is my fault alone which lead to her 'accidental' death. And knowing how faithfull a woman she acrually is would make me feel like an even worse person for providing a false accusation upon her.