You think nominal matters but in-actuality PPP matters. Market defines your currency rates, if you peg your currency at a lower rate then you have to inject $ to compensate for that, if your imports are more than your exports.
And about Afghan currency, there ain't any good other factor. The issue is these are all fiat currency, your market is what determines should be the rate of your currency( Afghanistan being an anomaly for it's currency being created recently so it's value is high). Here there was extra ordinary stress on nominal, which in reality should matter to the people living inside Pakistan, because when GDP is calculated it is calculated in local currency(which is PPP) then converted into nominal, with whatever rate. Now when rupee was 100 against $, PPP was same as when it was 157 (i am ignoring the 3% growth rate just for the sake of understanding) but the nominal GDP abruptly decreased from 320 B to 280 B. Also it is stated that our growth rate decreased from 5.2% to 3%, now where does it state that we have a negative growth rate. Negative growth rate tells you if the economy is is shrinking or not. GROWTH still mean grow not shrink.
Again I have to summarize it nominal GDP value was just a correction with the new value of Rs. Hope I explained it better this time.