Is it important that my NGO experience should be with a top notch NGO for MBA?

Most important is your track record of performance and promotions to more responsible positions. That said, NGO experience is not worth much in the business arena to begin with. Non-governmental organizations' requirements for "success" and "performance" bear little relation to the business world. Basically, you have no "business experience."

Work experience mans that you had a job in which you had to handle work responsibilities, made decisions, satisfied your bosses and customers or clients, hopefully advanced to more responsibilities, and gained knowledge beyond what you learned in your undergraduate program. It does not have to be business experience. On the contrary, an engineer would be expected to have engineering experience and satisfy clients; a politician would want to know how to run a campaign, attract voters, and raise funds, and a musician would be expected to have some understanding of what it takes to operate a music school, manage an orchestra, or conduct a performance tour. As you can see, everything involves business experience because the experience of living in an economic society is business experience. Only you can decide if your work with an NGO provided enough to have an MBA program admit you.

Don't use abbreviations that are so esoteric that people don't know what they stand for.