I agree from your point of view.
The mistake the writer made was never to introduce Radha well in her own background. She just barged into Mohan's life running away from home with remembrance of her crush over Mohan when she was a little girl. So, devotion was not pure but used only as a tool to enter his home. She volunteered to do good to Mohan and Gungun whom she came to know about only after entering his life. Mohan had to start liking her because of her forced good deeds. That is the reason why for initial viewers, Radha could never be accepted as a genuine character and her love for Mohan also. Mohan was forced to start liking her when she forcefully took over his life and Gungun's which was a contrast with her innocent character. I would place this fault entirely on the writer.