I can understand what you are saying, but from what I have seen of Rohit from day 1, he seems to be an extremely likable man, and not only Sona, any woman would fall for him if they get to know him. Do not forget that Sona has fallen for him only after realizing that he is a different person when he speaks his heart out and there is a lot of honesty and love with which Rohit deals with people for whom he genuinely cares including Sona.
Fashion show incident was a disaster, it was his sister for gods sake, and may be not directly but indirectly it was Sona who was the reason why Pooja went through this hideous humiliation. Sumeet's enmity with Sona ended up ruining Pooja's life, when she was not at fault. Obviously Rohit supported Sona after knowing the truth but the instant reaction he had could be justified and I would rather dislike him had he not taken this event this fiercely , a brother will never deal calmly or even practically when it comes to his sister's reputation, Rohit was impulsive and the situation was very much against Sona.
I believe he does not insult the actress Sona is, but maybe the way they provide everything on TV, which obviously is nonsensical and anybody would mock it. However initially when Rohit was a stranger for Sona, he mocked her show but slowly and gradually after learning about her POV and the hard work she does, he does not mock her anymore, his mockery for Parvati turned into a goofy kind of leg pulling where both are comfortable to taunt each other by making fun of their professions. It happens between friends, like we have seen how in FRIENDS, the friends made fun of monica's fake boob which she had to wear at her job place, making fun of Joey's acting skills and even Ross's boring lectures, these are given in friendships and if Rohit makes fun of Parvati now, only with the intention of pulling her leg not because he wants ti insult her or something.
I know what he is doing is not right and asking a favor from Sona to pretend as his GF, but why are we forgetting that Rohit has opened his heart out to Sona, shared how much he loves Raima and how he will wait for her, he loves to confide in Sona and he always believes that Sona can save him even when he is at his lowest, that confidence in a woman is rare, and Sona loves this trait of him and that is why she has fallen for him because he has given her too much space in his life. Also, he is the only one who pulled her out of her past zone and now she believes that she can fall in love again. Rohit does not know about Sona's feelings and i am sure when he learns about it he will feel guilty. Currently this is a mere natak for him which most of us can relate to. Even i have made my friends lie to my parents to save me, people do that to save themselves, of course things go ugly if there is someone as greedy as Suman and I hate the fact that Sona's mother does not care about her feelings at all. Still I really hope Rohit backs out from this plan and finds out a different way.
Last but not the least, what he did in the hospital was not showing attitude, but again his guilt and love for Raima, Sona claimed that he will never get Raima back and she has hit him where it hurt him the most. He apologized to her and also explained that how he cannot accept that Raima will never come back, that is his loyalty towards her and Sona accepted that gracefully, she understood his POV too.
PS: Karan is awesome as Rohit.. I hope the CVs do not change the characters for TRP sake.. what they did with Kasauti is horrible, what they did with EBSS is more horrible, I am scared about KHKT because i love this show and currently it is the only show which does not have any typical story line or villainous monster to plot against the leads.