Actually frozen vegetables is better to buy than fresh if you're buying in a store. Frozen is frozen as it's picked, while "fresh" has been picked, stored in boxes, transported to a central location, transported to the store, set out on display and then at whichever point you pick it up and take it home.
As far as whether a snake cares if it's prey was frozen and reheated to sufficiently warm to mimic "blood heat", I doubt it can tell or cares. There's a reason we do the zombie rat dance.... it's because pythons are usually fairly easy to fool.
The debate rages... frozen vs live... and it will continue to be debated ad nauseum with folks on either side who espouse ONLY their way. The rest of us in the middle watch with popcorn and shrug while saying "If it works for you..."