FOX (ended 2010)
Dale is actually quite brilliant when you think about (most overly eccentric people are in their own way) he has wide knowledge base of anthropology, artillery, federal law and public law his problem is he's very very paranoid. As for the whole his wife cheating on him. I think he's always known in his heart that Joesph wasn't his and Nancy has been cheating on him with John Redcorn all these years he just pushes all bad thoughts into his subconscious mind and makes up absurd stories to avoid the truth.
For example when Dale goes to the hospital to get his kidney remove while he's in the hospital bed sleeping he sleep talk and said "Nancy why is John Redcorn in the shower?" He has even told Hank he knows that him and Joesph look and act nothing alike (that was the episode when he came up with ridiculous story that alien had impregnated his wife with his own DNA thus the reasoning why Joesph was conceive while he was away. Sorry I don't know the episode name.) In the episode were Dale finds out that Joesph has a half sister, Nancy confronts John Redcorn about him cheating on her (even though that's the pan calling the cauldron black.) of course when Dale walk in she turn the conversation around and ask John Redcorn did Dale ever cheated on her. Dale knew that there was something weird about that conversation but instead of digging any deeper he decide to let it go and do nothing about the situation.
This got me thinking that Dale already knew the truth he just chooses to live in denial and who can blame him I mean which would you rather have to live in ignorance bliss were you have a hot looking wife who loves even though your just mediocre guy with tons of paranoid problem, and a loving son who think your the greatest or face the cold ugly truth that your wife cheated on you with someone stronger and better looking than you and while you were home raising a child that wasn't even your they were still going on their merry way have more sex. So no I don't believe that Dale is stupid he just lives in denial.