A currency having a lower value does not necessarily mean that the country is poorer or that the economy is not performing well. The value of a currency, in a floating exchange rate system depends on the supply and demand of that currency, which is related to exports, imports, transfers and even speculation etc., which means the value of a currency need not necessarily move in line with the GDP per capita of the country. Many countries also engage in dirty floating, where central banks actively buy and sell currencies to manipulate their value, usually to keep them within a certain range. This further obscures the relationship. There is also the once popular fixed exchange rate system, where the value of a currency with respect to another currency, a basket of currencies or gold is fixed.
In some cases, an undervalued currency can actually have some advantages. If your currency depreciates, then it means that foreigners will find your country's goods and services cheaper, thereby increasing their demand abroad, to the advantage of the sellers of your country. On the flip side, foreign goods will be costlier now, to the disadvantage of the buyers of your country. As with most of the things in Economics, this also comes to tradeoffs.
Some countries are alleged to deliberately keep the value of their currency low to encourage exports. For example, the US Treasury Department's semi-annual currency report has a monitoring list, which tracks currency market interventions, high global current account surpluses and high bilateral trade surpluses. In 2019, the US Department of Commerce said it would impose anti-subsidy duties on products from countries that it thinks engage in practices to undervalue their currency against the USD, possibly including China, India, Japan, South Korea etc.
In the long term, according to the Purchasing Power Parity theory, the value of one currency against another would be such that the same basket of goods costs the same in both the currencies. But this does not happen all the time, as the market exchange rate is more volatile and reacts to changes in demand in each location, the price of labour, tariffs etc. (see the Balassa-Samuelson Theorem). Even in the long term, trade barriers, non-tradeable goods etc. pose hindrances. The Relative Purchasing Power theory is a weaker version of it.
To summarise, the exchange rate is determined by a lot of different factors in different systems, and need not necessarily move in the direction of GDP per capita. Furthermore, in some cases, an undervalued currency may have some benefits.
