Just my opinion ... training and experience of U-Boot captains was part of the mix of May, 1943 but it's really the bigger picture.
There was a perfect storm working against the U-boots. May, 1943 was a year after the real effect of Operation Paukenschlag were apparent to the American naval authorities and the month marked a sea change in both the American and British (with associated Commonwealth allies) anti-submarine warfare. As the rough weather of the North Atlantic winter and early spring faded away, the U-boots were faced with more surface escorts, better tactics with sonar/asdic, radar, advanced radio direction finding, escort carriers, better coverage by shore-based aircraft ("closing of the Air Gap"), more Allied experience, newer weapons (hedgehogs, homing torpedoes, etc.), and Ultra.
As the weather improved, Doenitz sent his fleet of U-boots that had been either newly produced or repaired and maintained over the winter into a trap with multifaceted jaws. The entire situation was deadly.
And it wasn't just May, 1943. March 1943 saw the loss of about 14 U-boots and April about the same but most of the April losses (i.e. about 10 or a monthly rate of 20 or so) were in the last half of the month and leading up to May. (And there were many fewer U-boots at sea in March and April and thus fewer targets for the Allies.) May was certainly a major peak to a trend but that trend had been set on a rising curve for months.
So, while U-boat commander experience and training played a part in the events of May 1943, that part could only be described as a small part of an otherwise major trend.

HTH, B Henderson NC USA