Like Kathaa always says, Aarav is an integral part of her life, in fact he is her life. After Adi, for so long she hasn't seen or experienced life without Aarav even for a moment. So, what's wrong if the mother in her falls in love with Viaan first? What's wrong if the woman in her doesn't fall in love with him first? We all have different parts of ourselves within us, those parts take charge of our lives at different stages of our lives. For Kathaa, it's the mother in her who has been in charge for so long. In fact, I think only after the mother in her falls in love with Viaan, the woman in her can take forefront (because the mother in her has comfortably taken the backseat now) and take charge of her life, and only then she can truly move on, feel herself again, be herself again (as we see in the precap, enjoying herself, singing while cooking dinner, she is so chirpy even Aarav notices the difference).
I am not saying if someone is a mother, the woman in her should take backseat and the mother in her has to fall in love first or everything has to revolve around her child. But there is nothing wrong if it does happen and looking at Kathaa's backstory - loosing Adi, fighting for Aarav's life for so long - it makes sense. So, what's wrong with it?