Is it really necessary for a Nepali ( with a multi-entry visitor visa) to wait atleast 6 months inorder to visit US second time?

no

I think if you have visitor visa, you are suppoed to wait 9 months.

Let me know if anybody has entered within 6 mths with multiple entry visa.

I have entered within 30 days of my visit with B1/B2 multiple entry visa from Japan with Nepalese passport. Visits were February 27~March03, and the next one April 9~15.

There was no problem.

That was business visa


But what about visitor visa?

So far as I know, B1 visa is for business and B2 for pleasure.
B1/B2 is used for visiting relatives. They are also called "visitor visas".