I believe the P-38 was quite a bit better than the Zero, but not in every category.
The P-38 is faster, climbs better, has vastly superior systems and pilot comfort / conveniences, and is much more rugged.
On the Zero's plus side are a more reliable engine, better maneuverability, and lower wing loading.
The only aircraft that I am aware of that made better use of maneverability while being mostly slower than the opposition is the FM-2 Wildcat, which had an unmatched combat record in aviation until the F-15 came along just a bit later. It is likely that the engagements the FM-2 was employed in favored that combination seeing as how they were almost exclusively over water, implying Naval aircraft against Naval aicraft a LOT of the time, and that the Japanese pilots were much greener than their US counterparts when the FM-2 was employed. I am doubtful the FM-2 would have been able to continue its kill-to-loss ratio against the Luftwaffe over Europe.
All other slower but more maneuverable aircraft seem to have come out worse on the kill-to-loss end of things.
I have no doubt that, if properly employed, the Zero could hold it's own or better versus most competition, but it's weakness of having only modest power available dictated the other flaws, such as lack of self-sealing tanks, armor, and heavier construction. Given the modest power assigned to the airframe, the Zero was probably the best-performing fighter built during the war. I can think of nobody else who came even close with less than 1,200 HP on tap. Even the plucky little Sewdish FFVS J-22 had more power. The highest HP Zero model was the A6M5 Model 52, and it has 1,180 HP at full rattle. Almost all other models had less, and the prototype started out with only 880 HP!
Given what is known about proper tactics, I'd take a P-38 any day, but it DID take our guys some time to develop proper operating procedures and tactics to deal with the Zero, which coul be thrown about with little regard for stalling or concern over anyone following too closely.