Honestly, looking at the forum, I'd probably have assumed that GSD people were serious. Some people just can't take a joke and get their backs up about the tiniest things (insisting that someone is trying to take a dig at them, make unnecessary snarky comments etc. makes me think of them as nervy GSDs
Outside of this forum though, in the 'real world' I haven't noticed GSD people to be especially serious. Although I think every GSD owner I've met has been proud, and rather confident which I think is a great thing.
My own GSD is the goofiest dog I've ever met, so it's hard to be serious when you share a house with him.
Have I noticed any correlations with breed vs owner personality type? Hmm.. Golden retriever people are always really friendly!
In all fairness though, I'm guessing you'd find all kinds of people owning all sorts of breeds