The vast majority of targets you shoot in this game have no idea how to fight, so using Batfink or yourself as an opponent does not reflect on the actual ease of use. The fact is that the P-38J and even more the P-38L are very easy to succeed with. They have few vices and a lot of strengths. They are not as easy as the Spit XVI or VIII or N1K2, but they are easier than the Ki-84.
The P-38's great strengths compared to the rest of the LW set basically come down to its torque-less nature and its nose mounted firepower. For it to be "easy" to succeed with, you have to be seriously claiming that these two things are more important attributes in a plane for the inexperienced than turn performance, speed, and having multiple cannons, many airplanes having at least one and sometimes two of these things as an advantage over the 38. Hell, SpitVIIIs and SpitXVIs are just as fast and have superior turn and thrust/weight, Corsairs have superior speed and turn radius, late model 109s have all three advantages...
You put it a 38L in a multiple bandit scenario in the LW MA, and much more than virtually any other LW plane you have to really choose your situations, else you'll get in a corner where you are out-numbered and can't extend because the sky is filled with much faster planes and also dedicated turners waiting to get on you when the bnz planes force you to turn. Nor can it plunge into a 550mph dive to gain some breathing room like a Jug or Corsair. On top of that, the 38 is a large target, conventionally flown.
And your Ki-84 example? Have you gone *completely* balmy? I know I don't fly it in the MA, but I've got enough DA time in it to know how it handles. Its a poor man's SpitXVI. If the SpitXVI were not free, the Ki-84 would probably be one of the most popular rides.