There will be some variations of living standards depending on time and place, but for the average person the sixth-century standard of living was probably not much different from the fifth-century standard of living, for example. In some cases living standards may actually have improved because of lower taxation.
Of course, marks of civilization are more than just living standards of the common people. It’s definitely true that many of the refinements of civilization among the middle and upper classes were less in the 6th-8th centuries than they were under the Roman Empire. In that sense, civilization did take some steps backwards in those years before reviving.
I hope this answers your question.