The lens effect from something less massive than a star is too weak. The Sun has a focal point 542 AU away from its center; our moon would have a focal point so

The lens effect from something less massive than a star is too weak. The Sun has a focal point 542 AU away from its center; our moon would have a focal point some orders of magnitude beyond that, and the angular resolution wouldn't be nearly as good. In short, it's far easier and more effective to use the Sun for this effect. Radio interferometry would be a better use for Moon-based observatories, if we felt like building several of them.