Mantes are not poisonous - but that isn't to say that any wild caught specimens did not come into contact with some kind of chemical -cide or consumed an insect that might itself be toxic. Both of which would be unlikely though, in my opinion, as the mantis itself would have probably succumb to either before being eaten by the frog. As a VERY general rule that should not be taken too literally; predatory, cryptic insect species that are heavily camouflaged are not poisonous.
Just my $0.02 - Heather and Lynn have provided some great information and questions to help us further along.