With nearly half a million Google results, I think we can assume that prepped and ready has entered our language as a phrase, whatever the redundancy implications.
If I wanted to make a distinction between the words, I would suggest that prepped means that you're prepared with all the necessary tools you need, and that ready means that you are mentally prepared as well. But I'm making that up just for you.
I think "prepped" means that you have finished your preparation (or perhaps you have been prepped by somebody else" and "ready" means you are now ready. So there is a slight difference. The prepping is an activity that took place in the past and is now finished. Being ready is more a description of your current state of mind.
Nobody stops to think about any comparison or differences. The phrase prepped and ready is idiomatic. Used separately Prepped would mean that you have done everything to avoid a shortage. Ready would mean that you can begin.