Strength is very important. But the strength required for playing the piano is not what many people think of as strength. It is not the strength of muscular tension and exertion. We're not weightlifters.
Piano playing requires inner strength, which comes from, somewhat paradoxically, the relaxation of the muscles involved. Proper coordination of those muscles is what constitutes strength.
Everyone has the ability to harness their inner strength as much as the next person. So, provided the pianist has two arms and is not badly malnourished, strength of the muscles themselves shouldn't matter too much. Bodily constitution will make a difference, but will rarely be a limiting factor.
- Saturn