What cartoon is about a dog that eats mustard? - briefly
The cartoon you're referring to is likely "SpongeBob SquarePants". In an episode titled "Mustard O' Mine", Plankton attempts to use mustard to lure SpongeBob's pet snail Gary, who loves mustard.
What cartoon is about a dog that eats mustard? - in detail
The cartoon in question is likely "Curious George," specifically the episode titled "Mustard Sandwich." In this beloved children's series, based on the books by Margret and H.A. Rey, the titular character, George, is a curious little monkey who often finds himself in humorous situations due to his insatiable curiosity.
In the "Mustard Sandwich" episode, George accidentally gets mustard all over himself while trying to make a sandwich. This leads to a series of amusing events where George's friends and neighbors try to help him clean up the mess. The episode is filled with the typical humor and warmth that characterize the "Curious George" series, making it a favorite among both children and adults.