To start, I think the "complaints" or confusion about why we would care if he's a hometown guy are rather strange. Isn't picking the team close to you the same thing? Isn't that why nearly everyone across the country cheers for the USA in the Olympics? Isn't that why people from Wisconsin are the "border rivals"...but then if they come play four "our team" they become "local" guys? I don't think this is a phenomenon at all unique to Minnesota, although there may be differening degrees in various regions (and if there are, I'd guess the midwest ranks high on that list...wait, did I just...?).
As for booing...Mauer or anyone else on "your own" team...well, it's a subject that gets me pretty riled up. Maybe I should just move on to the next topic. But here I go... I'd say with some exceptions, if you're booing someone on your team for failing, it pretty much makes you something that rhymes with brass bowl (apologies to Simmons for stealing that). The first rebuttal to that is usually, "I/they pay their money, they have the right to boo anyone they want." I agree. Just because something is your "right," doesn't mean you're not a brass bowl for exercising that right. As I said, I think there are exceptions: clear lack of effort (and probably repeated) might be one, something like that. I could go on and on, but I'll just end up ranting...