Talking of Harvard, has there been any Nepali who went to harvard and then went on to contribute something big for Nepal or the world for that matter?

Not that I know of.  I know one HBS grad who is a VP at a reputable hedge fund in emerging markets.  I know of another Harvard grad who is at World Bank on Latin America section.  As for their contribution to the world, I really can't say.  

I know of grads from other schools who are making significant contribution in Nepal and around the world.  And there are grad(s) from Harvard who like to talk big, sell themselves with their tenuous ties to Harvard, but hardly have anything to show.

This does not mean that Nepali Harvard grads are losers.  They might be.  They might not be.  But does it really matter?