Yes, it's normal and they're just the seed pods from the spent blooms. I have a mature lilac hedge of maybe 150' in length and I never bother to cut spent blooms off as they add some winter interest once the leaves drop. (And it would be too monumental a task anyway.) Plus they disappear in the spring when the lilacs put out their new growth.

It's a little late in the season to be cutting on lilacs, you may accidentally cut into the growth that produces next season's blooms.