Earlier quoted context omitted.
I mean, there definitely seems to have been a large decline in population, economic output, political cohesion, urbanization, record keeping, trade, etc., in much of the former Roman world following the fifth/sixth century. I suppose it's a value judgement on whether one considers that a "bad time."
The dark ages also saw a huge disparity in education between the East and the West. Until the 15th century less than 5 people in Europe could do long division. For comparison, Aryabhatta discovered calculus in 5th century India and Madhava made large contributions in the 13th century (Newton/Leibniz were wrongly credited for similar work many years later).
That’s wildly inaccurate. Arithmetic was very much a subject at several schools with the addition, subtraction, multiplication, and division all being taught. https://en.m.wikipedia.org/wiki/Quadrivium
Long division being difficult in Roman numerals is one of the reasons for the switch.