WASHINGTON — After Monday it may no longer be possible to slip between the two enormous flippers that frame a doorway in a shopping mall in Georgetown and bounce around in a strange world of sleekly raked playing boards, gleaming silver balls, rubber-wrapped pins, jittery bumpers and raucous bells. It looks as if the National Pinball Museum, which opened here only in December, is about to close, almost before an out-of-town critic could offer it the ringing approval it deserves.