After carving out a reputation as the Reagan administration budget slasher, budget director David Stockman now says the budget-cutting program was poorly planned, hastily enacted and ignored the Pentagon's ''blatant inefficiency.'' ''We didn't think it all the way through. We didn't add up all the numbers,'' Mr. Stockman says in an interview in the December issue of The Atlantic. ''The reason we did it wrong - not wrong but less than the optimum - was that we said, 'Hey, we have to get a program out fast.' And when you decide to put a program of this breadth and depth out fast, you can only do so much,'' said Mr. Stockman.An Office of Management and Budget statement said that the Atlantic article created a wrong and misleading impression, and said Mr. Stockman still believes in the President's program.The magazine profile of Stockman highlights his efforts to slash the Defense Department budget and his growing discontent with ''supply-side'' economists. ''I'm really going to go after the Pentagon,'' Stockman said. ''The whole question is blatant inefficiency, poor deployment of manpower, contracting idiocy ...'' The article said Stockman is exasperated about not being able to do more as head of the Office of Management and Budget. Stockman said the supply-side theory was merely a version of ''trickle down'' economics, in which tax cuts in upper brackets are supposed to trickle to lower levels. He characterized the Kemp-Roth tax reduction proposal that candidate Reagan championed as a ''Trojan horse'' that ultimately enabled him to cut taxes for the rich. The White House Wednesday declined comment on the interview.