If there is strategy wherein you can make yourself stronger by taking damage, couldn't bluffing make sense? You drain your opponent of a strong monster and get a lot of cards at the same time. Sure you may lose some cards in the bluff, but if they're all useless anyway, why not speed up the process of getting good cards?
I watch the series trust me there are worst examples than you have given. There was an episode When Keven bit gold feces to check if it is real gold. That is an easy set up I gave you. However they can get away with a few examples you have given. Remember the Omnitrix is meant to be a communication device. They can get away with similar species. His past self said the same thing you said. His kid form had the best lines in that episode.