Is it okay/good to ride in a thunderstorm? Well, that depends. Since lightening strikes the highest point, are you on the tallest horse or is your friend you're riding with on the tallest horse? LOL! Sorry couldn't resist!
I absolutely do not ride in thunderstorms. I do not kayak in thunderstorms. I do not do much of anything in thunderstorms. Lightening scares the bajeezies out of me!
Riding in the rain is fine. I did get caught once up north, with friends , riding up a mountainside and suddenly all H*LL broke loose. Wind picked up, dark clouds, thunder, skies opened on us and than, gulp, that flash of lightening. Turned right around, put T in an extended trott and off I went. Didn't take long for the others to turn, but only one caught up to me and T and they had to canter to do it. Yup, I have very strong survival instincts! Didn't faze T, but me, absolutely.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".