Question

Why would it be bad for a mammal to have a right ventricle that pumped blood at the high pressure typical of the left ventricle?

Answer 1

The left ventricle pumps blood at such a high pressure because it has to reach the entire systemic circulatory system.
The left ventricle also has thickened walls to withstand the force

The right ventricle only pumps blood to the pulmonary circuit. A high pressure could be damaging to the delicate structures in the lungs as wells as the right ventricle does not have thickened walls to withstand such high pressures.