Mallu that title gave me all the wrong ideas for a moment there 🤣 🤣
I was like damn what did I miss 😲 🤣🤣
But coming to your post - I suppose Raman is bummed out because he thinks Mihika isn't the right girl for Mihir since she too believed during the MMS scandal he was guilty. He thinks love should be implicit and faith unshakable. He knows Mihir for the person he is and trusts him unquestionably - that for Raman is a BIG deal, he doesn't trust easy, and he felt if Mihika cannot trust Mihir, cannot vouch for his innocence - she doesn't deserve him.
Now what remains to be seen is how will Ishita tackle this issue, with Trisha' mysterious lover cropping up out of the blue, this will become murky. Trisha so far gave the impression she couldn't be happier with this marriage and relationship but with the shocking discovery of her BF it would shake Raman up and maybe that is when Raman will realize the girl he chose for his bro would have ended up ruining his life had this marriage happened. He would have unwittingly made Mihir a victim of his past and found his history repeated in his life.
The bright side of this random guy popping up is now Raman wont suffer a loss in business by losing his biggest investor in Tandon - if his own sister is duping everyone and thus in breaking up the engagement of Mihir Trisha, Raman wont be able to blame Ishita for long or stay mad at her forever for no reason. It would become clear to him Ishita in fact helped everyone here and a major problem was avoided because of her timely intervention.