Just so you know, some of us, in Python, do iterate over indices and not modified objects when it is helpful. Although in NumPy, as a general rule, we don't explicitly iterate unless there is no other way out: for your problem, the simplest approach would be to skip the iteration and rely on broadcasting:
mnX += cnA
If you insist on iterating, I think the simplest would be to iterate over the transposed array:
for col in mnX.T:
col += cnA[:, 0].T