Making it up as you go along isn't inherently bad. Nine times in ten I prefer a story which is planned out but basically any medium that's open to additional seasons, novels, sequels, etc is capable of falling into this category.

It's only really a sin when the medium promises a long form mystery while doing this, hence the fact Lost is #1 here. Sherlock Holmes was written as episodic mystery and Arthur Conan Doyle clearly never planned future stories as he went and nobody minded. Togashi, the manga author for Hunter x Hunter stumbled into his most famous arc just because he'd made his metaphysic and societies up as he went and the stars aligned, leading to the Chimera Ant arc. The Simpsons rarely ever changes it's status quo between episodes, and therefore can be made up as it goes along, because it's going nowhere. Breaking Bad literally changed the ending of season one to not kill Jesse partly due to the writers strikes and subsequent shortening of the season, and Mike as a character exists because Bob Odenkirk was busy.

Any medium that decieves the audience, promising a well reasoned, long form mystery without any planning of what that mystery is, is bad. Perhaps you'll strike gold and have an epiphany as to how to bring the plot together perfectly, but that'll just be luck. Ultimately this is an expression of consumerism; baiting the expectations of art and narrative to deceive the audience for nothing more than engagement, and therefore money.