Is mild left ventricular hypertrophy bad?

It is abnormal but not that bad. I assume you have hypertension which was out of control for quite some time. If you are taking medication(s) for controlling the blood pressure, keep it below 120/80, and if your doctor uses an ACEi or ARB, the heart muscle can be stabilized or get smaller (re-conditioned), i.e. the hypertrophy will not get worse or it even improves.

If you indeed have left ventricular hypertrohy, it is indeed abnormal and warrants examination by a specialist. I assume you do not have hypertension which is the most common underlying cause. I would certainly get additional evaluation. Remember that an electrocardiographic diagnosis may not be reliable. I urge you to follow this up.