Is a block in the right bundle branch serious?

No: Millions of people have rt bundle blocks, heart function remains normal. As you age, other conduction issues may arise, and of course blood vessel disease, etc.

Generally not: Right branch block is, frequently, caused not by complete block in the bundle but by a delay relative to the left bundle. In this case, there is no risk at all. There are generally three fascicles which supply conduction to the lower chamber. The right bundle is only one of these three, and even with complete loss of the rb, there are still the two fascicles of the left bundle remaining.