I agree with you about the Zimmerit, and it's a good example of the dictum 'forewarned is forearmed', in this instance, having no tactical value. Zimmerit was really a colossal waste of time and material, and it also added unnecessary weight to tanks daubed in it. BUT..it is fun for some members to apply to their models.
Given the curve of the chinless Panther mantlet, there are numerous points below the horizontal were a deflected or ricochetted round wouldn't have to travel at 90deg. The 6- Pounder round that hit Tiger 131 got wedged between the turret and the upper deck, for example (yes, it was a different model..

, but a similar principle).
More importantly, at anything other than close range, the round would be
descending in a parabolic shape anyway, meaning that actual deflection would be minimised, as well as the loss of latent energy.
As an aside, it's not even necessary to penetrate the roof of the tank, nor other plates in order to kill the crew inside. HE and HESH rounds could do that. Crewmen could be minced by the 'spalling' effect.

- Armour cross section showing beginnings of spalling
- Cross section of armour with evidence of spalling.jpg (92.69 KiB) Viewed 1780 times
Given the generous the inward slope of the Porsche turret, a round would be neatly directed through the roof, or into the turret ring mechanism. In either case, it would have been an unwanted outcome for the Panzer crew.
Finding evidence for all of this is tricky, though; but that's part of the fun of being (historic) Tank detectives.

"Get your facts first, and then you can distort them as much as you please"- Mark Twain.