I don't think the bias is nearly as 'blanket' as you like (or need?) to think. I don't think there is any blanket assumption that a person who sees a UFO is necessarily drunk, stupid, uneducated or insane at all.
Really? OK, before you rush to make any judgements, why don't you try a little experiment all by yourself.
Say something like "I saw something really odd last night, ... I don't know what was .... but I believe it was not of our world". Or, anything else like this that you think would work, I'll give you full creative licence here. Then watch for the reactions. Some will be positive and inquisitive, while a great many will be rude, and will either insult you to your face, or attempt to deride your character behind your back. Just a little experiment .... go ahead.