Good grief he was damn funny. He couldn’t play straight it just wasn’t in his genes. The moment he came on camera you knew you would be laughing soon. Best in Show, Waiting for Guffman and A Mighty Wind we’re a few of my favorites.
My perception was that he was playing it completely straight in one of his SNL sketches. He was a director of a movie where Bill Murray was a gangster holding a woman hostage when he got shot. Belushi came in to do the stunt, and first he whined about something minor, then he had a major problem which Fred helped him with. Then he fell to his death, but Fred was completely professional in his reaction and told everyone to prepare for continuing the filming. If he was supposed to be funny, it wasn't obvious. Belushi was getting the laughs.
When he first came out, though, it was not for a monologue, but to sing an Elvis song (I assume) dressed as Fat Elvis complete with cape. When he finished he took off his sunglasses and said, "It's really me!"
Another sketch I found forgettable had Fred and his wife running a mall store selling nothing but scotch tape. Not recording tape, but just the sticky kind. People came in wanting other products. The wife got upset because she was afraid the business was going to fail and Fred encouraged her, but I couldn't see anything obviously funny except the concept of a store selling one product.
The final sketch was somewhat funnier. Fred was a pastor who had his program on just before a TV station signed off at midnight. He explained that there was once a time when people believed in many gods. We saw the first man who had doubts and began to believe in one God. Belushi heard the voice of Don Pardo and went on to tie up his son, Bill Murray, in order to sacrifice him. Back to Fred, who said his wife was frustrated and looking at other men because his job wasn't that prestigious and he had to take a second job. Belushi started attempting to stab his son, every time hoping God would stop him. The son made him made and Belushi went through with it, only to hear Don Pardo yell, "Wait!" Fred explained that God had more trust in man later and the outcome for Abraham would be different. And he told his wife he was on his way home and she'd better be alone.