Isn't that kinda true, but mostly because of the situation they found themselves in? Shermans in Normandy were forced to do the duty of heavy tanks (breakthrough) despite being designed as mobile medium tanks. Meanwhile, Panthers were in a defensive situation where mobility wasn't that important, meaning that they could capitalize on Panther's good aspects (excellent gun, good frontal armor) while not being that much held back by Panther's lack of reliability and disadvantages it did have.
Plus, I think early Shermans had some issues with ammunition stowage and tank bursting into flames when hit, which were solved later, but created a lasting perception of M4s being only good at roasting their own crews. But it's been a while since I've read about it, so don't take me at my word.
Agreed. Although hp/ton is still important because it tells you how much weight you can have with a given engine, or else how powerful of an engine you need for a given weight.