I am an unapologetic Appaloosa fan-- pretty much all types, patterns, bloodlines-- I love 'em!
I can understand if people don't like whatever various appaloosa coat patterns, pink/mottled skin, thin tails, or are not a fan of their "generally" stock horse conformation (although there are plenty of Appaloosas with "normal" to heavy manes and tails, not much mottled/pink skin, etc.... there are a ton of possible colors and patterns, including solids.... and Appaloosas do come in a wide variety of body types)
That said, I think there are a couple of reasons that people get a bad "overall" impression of Appaloosas besides the above--
One reason? Generally, Appaloosas are pretty affordable, and their color attracts people. Unfortunately some people breed for color without breeding for other more important qualities. This creates some poor examples that can be bought even more cheaply, and then more people who don't want to spend any money buy these, breed them, and the cycle continues.
Another reason?-- color recognition. Many people consider anything with appaloosa coloring an Appaloosa. Color is just one Appaloosa trait-- The horse might be a mustang-percheron-walking horse-shetland cross (nothing against any of those breeds, but you can imagine the train wreck if they are all carelessly thrown together in one horse, LOL) but slap spots on its but and its suddenly an appaloosa! I guess just like some folks will call any chunky big-hipped solid chestnut a quarter horse....
And, because people remember those spots, IMO they are more likely to remember the bad ones. If someone rides 10 stubborn ill-tempered chunky big-hipped chestnuts in a row, They probably wouldn't think too much of it-- and wouldn't start to think that all Quarter horses were stubborn or ill-tempered.... but if they rode 10 stubborn ill-tempered horses with spotted butts..... they most likely will start to think all Appaloosas are a problem!
Again, I love 'em-- here is a sampling of Appaloosas I own or have owned, have bred, or offspring of stallions I have owned..... a wide variety--