The dub of Porco Rosso is a rapidfire of terrible acting, ridiculous junior high school quality affectation, and shockingly awful miscasting that easily fits in the same dumpster with Castle in the Sky. The use of name actors is not always a solution. You can't cast just any known actor and expect them to fit the role. Michael Keaton's acting is fine, but he's far from suited to the role of Porco Rosso. The incidental voices of the rest of the characters are either typical anime dub cheese garbage, or worse. Some of them are so bad it feels like they just picked up a random moron off the street, offered them a sandwich as payment, and told them to do the stupidest voice they could for their assigned role.
It's pretty obvious why Princess Mononoke and Spirited Away got theatrical releases, and the others didn't. There are good performances in Mononoke Hime and Spirited Away, although they're far from perfect. Compared to those, Castle in the Sky and Porco Rosso are regurgitated offal. Kiki's Delivery Service is somewhere in-between, combining the odd casting choice of Phil Hartman with the relatively good choices of some of the other character voices.
Actually, the best dub I've seen in a long time is the dubbed version of Detective Conan, shown on Cartoon Network under the title Case Closed. It's not perfect, but some of the performances are actually pretty decent. Whether that's thanks to the director or actors or both is something I have to wonder.
Overall the anime dubbing industry is still in the dark ages, and continues to crank out unprofessional crap like Yu-Gi-Oh! and Zoids. Fanboy apologists will jump at the chance to say that television anime somehow doesn't count, and that you shouldn't expect good dub work out of them. They're wrong. This is America. The most popular and some of the best television and film entertainment in the world is made here. This is where Warner Brothers, Disney, and MGM defined animation for the whole world. This shameful situation should be an embarrassment to the whole nation. We should expect better.