Alright: facts!
Fact: Snape is written better than Sirius.
However, Snape also has four more books to be written better than Sirius in, and in Book 4, Sirius plays an extremly minor role. So of course a character with essentially quadruple the screen time is going to get far more moments to shine. That's not Sirius's fault, it's Rowling. She didn't write him as in-depth as Snape because I'm assuming she knew beforehand she was going to sacrifice him a lot sooner than Snape.
Anyway, in the short amount of time he is given Sirius does a spectacular job of being a compelling character (to most), and a likable father-figure for Harry. It's made glaringly obvious that he loves the boy like no other and has willingly risked his own life on several occasions and sacrificed his own health over a long duration of time to make sure Harry stayed safe. He loves Harry as if he's both his own son and his own best friend, and no, I'm afraid I don't find this behavior "creepy". Perhaps if there were even one hint of sexual connotation by Rowling I would find it so, but as it stands Sirius just seems to treat Harry like "an old friend", and Harry treats Sirius as "the closest thing to a parent" he has ever known- I believe these exact phrases are used to describe their relationship.
Regarding his immaturity, Sirius rejects his family's beliefs not out of some petty squabble or "a rebellious streak", but because he's firmly against pureblood elitism and widespread usage of the Dark Arts. His treatment of certain family members absolutely reflects that his opinions are not just some act. Perhaps the posters of bikini-clad Muggles on his walls were overkill, but can you really fault him with proudly displaying his beliefs? His mother disapproved of him as soon as his peculiar tastes became known anyway, so really, what would be the point in even hiding it? Fear of being disowned? He would probably look forward to no longer being associated with that woman!
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