The M-3 Stuart was actually pretty popular in North Africa where it was found to be fast and mechanically reliable. The Brits called the “Honey” for that reason. The 37mm gun was adequate for the period and, unlike the British 2 pounder, had a useful HE round. At the very least the 37mm was no worse than the 2 pounder (40 mm) AT gun that British tanks like the Matilda and Crusader carried.
The “underpowered” 57mm was the British 6 pounder and both the Churchill and Valentine tanks were using. They were certainly a big improvement over the 40mm two pounder and were marginally better against armor than the low velocity 75mm that Shermans of the time carried.
Ya beat me to the punch!