Before pulling the ripcord
Skydiving through rain can be painful because you’re falling at terminal velocity, so the rain is driven into your face at up to 200km/h. In a thundercloud, your chances of being hit by lightning are higher than on the ground because your wet body presents a more conductive path than the air around you. Nearby lightning bolts will jump to you, then continue to the ground. This happens to planes, but the passengers are safe because the metal skin of the plane diverts the bolt around them.
After pulling the ripcord
If you try to parachute through the thundercloud, things get much worse. The turbulence can tangle your lines, or wrap you in the canopy, and the updraughts can mean you gain altitude instead of falling. You could be flung so high that you asphyxiate in the thin air, or freeze to death. In 1959 a US Marine pilot called William Rankin ejected from an F-8 Crusader fighter jet directly into a storm. It took him 40 minutes to reach the ground and he suffered frostbite and severe decompression injuries.