Is cardiac fibrosis bad?

Yes, the fibrotic portion of any organ will lose its normal function. So, if a portion of the heart muscle becomes fibrotic that portion of heart tissue (myocardium) loses the ability to contract effectively so the heart is weaker.

Bad is a wide term. But yes it can be fatal which I call bad. But may start not so bad and work up. Some causes can be treated to stop/reverse it such as hemosiderosis. So you need to find cause to get best treatment. Discuss with your Drs.