Yes, the current cargo activity is a direct result of the Scottish Government’s decision to buy the airport.

In 2013 the Scottish Government bought Glasgow Prestwick Airport from Infratil for a nominal £1 specifically to stop it closing. Without that intervention the airport would almost certainly have shut. https://www.bbc.co.uk/news/articles/cqxq8l3v984o⁠

Under public ownership it has been turned around and is now profitable. Cargo has become a major part of that recovery: volumes have risen sharply (the 5,548 tonnes reported for August is one example), driven by new daily services to China and other freighter operations. That growth has created jobs at the airport and in the surrounding aerospace cluster. https://www.ayradvertiser.com/news/26648168.prestwick-airport-cargo-rises-5-548-tonnes-august/⁠

A preferred bidder later withdrew from a sale process, so the airport remains in public hands for now, but the cargo performance and operating profits are outcomes of the 2013 rescue and the strategy pursued since.

Was the decision to buy the airport criticised?

Ho ho, that great all-rounder Martin Williams, revealing his dim-witted loyalty to the opposition party dim-wits.