Mathematically, sloped armor is more efficient. Yes, sloping a piece of tinfoil isn't going to be as strong as a flat piece of cast/hardened armor. If you take two equivalent plates, place one flat, and place one sloped, the sloped one will offer better protection. The round has to travel through more metal and its tendency to curl off is greatly increased.
You're not taking into consideration that the German tanks facing the Sherman were facing a tiny, worthless infantry support gun while the Sherman ran into nothing but "tank destroyer" guns.