After imposing the sanctions to Russia, 1 USD skyrocketed to 139 Rubles, but now it is back to the pre-war exchange rate. Moreover, it kept decreasing almost every day recently. Does this mean that the Western sanctions on Russia did not work as intended and the Russian economy will not be hurt by them?
No, it does not mean that. Currency value just reflects supply and demand for the currency. It is not a measure of how economy is performing.
There can sometimes be some correlation between exchange and how economy is doing but exchange rate fluctuations are not good measure of economic performance. If an economy is in a turmoil people might want to move their money out of the country and in that case you can see exchange rate drop being correlated with health of the economy. However, the drop in exchange rate is caused by people purchasing foreign currencies and selling rubles, which can occur even when economy is performing well. Furthermore, Russia has capital controls and consequently there might be some people who want to sell rubles but can't so in this case even economic turmoil might not cause exchange rate to change much.
To see how much economy gets hurt by sanctions we have to look at some real variables like real output or similar measures of economic performance. There are no data on Russia's output, after the sanctions released, yet. These things are typically released by 2-4 quarter lag.
However, economic projections by respectable economic institutes such as the Institute of International Finance forecasts that Russian GDP will contract by about 15%. That is more than Russia contracted as a result of Covid19 pandemic. Hence if these projections come to be true the sanctions will hurt Russian economy considerably.