lol what? that thing is easily one of the best medium tanks in the game... high armor, great ai and adequate at to fight most allied tanks head on.
You are missing the point. How good PzIV J is has to with balance of the unit and it did not claim it was bad, what it bring to the table has to do weather it is worth having a commander slot for it.
The question is worth buying over PzIV? Even if it is could it bring more to the table so people have more reason to build it?
If you compare PzIV and PzIV J at vet 3 the first one is slightly more mobile and has better rear armor(!!!) and the later have better scatter values that basically make better vs infatry.
The unit can be designed to bring much more to the table.
if you want to engage heavies then a panther is obviously the better choice, but for anything else the pziv j is the best ostheer has to offer.
If one want to engage Super heavies Panther should be the choice but Panther is not very cost efficient vs heavies like KV-1, Churchill...
Designing the PzIV J like 76mm Sherman with higher penetration values and lower AI would create a unit that would have a more specialized role.