Definitely not in the way you just described it, because I had to read your question 3 times to understand what you just meant.
If you are asked your opinion on how to reduce the car accidents rate – then yes, it's a valid question to ask how this rate is calculated (is it car accidents per thousand of people, car accidents per thousand of drunk drivers, or car accidents per under-aged drivers – those 3 rates are very different and look at different problems).
The same is with any "rate". Population decrease rate of course has a numerator (death or move-out) and denominator (birth or move-in) – so, yes, definitely you need to understand what those are.
The best way to ask this question is to say: "Could you please specify how this rate is calculated?"