At the mean position of the SHM the velocity is maximum. It can be proved by law of conservation of mechanical energy.
At any point of the SHM,
Total Energy= 1/2 mv2 +1/2 kx2
(where 1/2 kx2 is the Potential energy of the system)
For maximum velocity, kinetic energy should be max and hence potential energy should be minimum. So applying some simple logic:
1/2 kx2 =0 and therefore “x=0”. Hence the mean position.