It would have been a far more effective tank, at least in my opinion.

The Tiger I, for its time, was a really great tank. While some people argue that it suffered to greatly from mechanical issues, similar to what it’s successors would experience, this isn’t really true. In fact, the Tiger was, typically speaking, a reliable tank and frequently praised by its crews.

If the Tiger I featured sloped armor, it would have been an even stronger tank than it already was. Assuming it’s 102mm frontal plate was constructed to a 55–60 degree angle (similar to the Panther tank) rather than 90 degrees~, it would have an effective armor thickness of 150mm, at least. This would be enough armor protection to resist close-range hits from the Soviet 85mm D-5T, the American 76mm M1, and the British 76mm 17-Pounder. The Tiger would even be protected against strikes from the Soviet 122mm D-25T, which was capable of penetrating Panther tanks around medium and close ranges.

– Testing on captured Tiger tanks by the Soviets. The target was hit, multiple times, by the 122mm A-19 gun (predecessor to the D-25T).

If this change was ever implemented, it would have basically rendered the need for a new tank like the Tiger II non-existent. The Tiger I’s 88mm KwK 36 was already potent enough to destroy anything the Allies had by 1942, and it would stay that way up to 1945. German engineers would also end up increasing armor to the turret, although it was already decently well protected to begin with.

However, the armor plate being at an angle would increase the weight slightly, so it’s possible that more strain would be placed on the Tiger I’s drive train and transmission. I doubt the difference would be very noticeable, but mechanically the tank might suffer.