If interesting, different, and fun = bad, you're even worse than the slimes!
You play as a silent character that does not develop and if someone wants to complain about that, go look at Zelda for one. Are you going to tell me that because Link's personality did not develop during Link to the Past it's a bad game?
Anyway, you are recruited by the space army or some nonsense and you must rid space of the slimes that hate advertising (hey, I laughed). You must first defeat spaceships in an Asteroid-esque mini-game then warp to the planet and take out any number of slimes. If you saw the quirky art-style of this game and thought taking on this task would be somehow "epic" you'd be the only one to blame. This task is usually simple, but with every world you take on different puzzles and gain new abilities and items. Figuring these things out may be a cakewalk at times, but it's rarely ever a true bore.
The small amount of other human characters you meet in outer space are pretty hilarious and make you almost want to side with the slimes at times.
At the end of the day, Big Sky Trooper is funny and different and works in all the right places. How does that make it a bad game?