Most splash whites have colored ears, I don't think deafness is the cause of the stereotype, deaf splashed whites generally tend to be really loud (so easily recognized as a splash), not always of course, but generally and splashed whites don't carry the stereotype of being crazy. I think it's just one of those things that happens, same as the idea that white hooves are weaker. So far I haven't had a crazy blue eyed horse, not saying there aren't some out there just I haven't encountered one (but it wouldn't be because of the blue eyes), don't know that any have been splashes, although I suspect my current one is.
I've heard alot of horror storries about deaf dogs, they are ticking time bombs, they will eat you alive if you touch them while they are sleeping (or not looking), you can't have them around kids, etc. guess what, my deaf dog is nearly 8 and the gentlest thing in the World, I have 15 nephews and one neice (soon another nephew and neice), that have all grown up w/ her and she hasn't eaten one yet! The majority of the poeple that are so sure deaf dogs are dangerous have never had or been around one, and most usually get that from other people w/ no experience and breeders that want to justify having them put down at birth. No one even has a clue she's deaf til I tell them.
I think it's just one of those silly things that once it gets started it will never end.