Hmm, can you elaborate/tell me more about on this "bad foliage" or post additional pictures?
I did not see anything "bad" on the previous pictures. There are some leaves that have produced dark reddish fall foliage on the last picture but that is ok.
At this late point in the growing eason, some mopheads reduce chlorophyll production and when the production of greens in the leaves is reduced, you usually see yellows. A few lucky mophead owners have varieties whose leaves have more orange & red pigments and they turn nice fall colors at some point in the Fall. I have a bunch of oakleaf hydrangeas and these are famous for their fall foliage most years, although it has been so rainy here that I am not sure what will happen this time around.
Also of note, mophead hydrangeas always develop invisible flower buds at the ends of the stems between July thru September. The exact time varies a lot and depends on things like whether it is a rebloomer hydrangea, current weather, soil minerals and where the plant is located. If you are going to keep the plant inside or if you are going to winter protect the plant then the stems may survive your winters and bloom early in Spring. So keep those pruners away or else you will be cutting off the invisible flower buds that open in Spring 2019. If you want to cut anything, cut the leaves or blooms at the petiole string that connects the leaf/bloom to the stem; do not cut the stem.
If this shrub is a rebloomer mophead, it will develop new stems in Spring. These new stems will develop a flower buds and a new flush of blooms too. You have to wait for these new stems to get tall and old enough to bloom. That happens around July-September.
By the way, I have not asked but do you know if this is a rebloomer mophead?