Don't know about if one should get a smaller frame. However due to their design, the measurements are different for a MTB as compared to a road bike. Usually you will fit a mountain bike whose frame measurements are 9-11cm less than your road bike.

So if you ride a 58cm road bike, you will likely best fit a 47-48cm (or 19") mountain bike.

Same is true for a hybrid bike. I usually ride 17"-17.5" hybrids, which is 43-44cm. But I fit a 52-54cm road bike.