Father Dougal has schizophrenia
Think about it: he is completely detached from reality while nevertheless occasionally saying and thinking things that do conform to reality to varying degrees. And he often does things that are not outright stupid but just plain weird and nonsensical. And he only became ill after becoming a priest.
I know the show takes place in an over-the-top cartoonish universe, but I think this explains Dougal's...Dougalness, he he.