I think part of the problem is that media has trained us to think of super geniuses as either absent minded autists or smarmy jackasses. The idea that a normal seeming guy could have an IQ of 170 becomes difficult to find believable.
I was going to go into some detail here, but cutting it down severely, I know a lot of people with very high IQs (and not the "I did a test on the internet" kind) and have a fairly high one myself and I have to say, the stereotypes are broadly true (probably of me too!), albeit almost everyone mellows with age and experience (up to a point, at which point they sometimes worse). This isn't a stereotype based on a kernel of truth, it's based on a solid, tangible lump of truth!
But there are exceptions, that is for sure - I can immediately think of one person I know IRL who is that smart, that educated, and proper scientist (and kinda famous, many of us have probably ready some of their work) who is an extremely nice, warm, kind, decent person, more so than most people (and only about as socially awkward as Ruffalo's Banner).
But I think think the issue here is more specific to this MCU version of this character.
Like any character, there's been a million takes. The old Bill Bixby Banner wasn't particularly angry at all. Heck, he wasn't even all that repressed. But, his trigger was usually because of the relentless hounding and pursuit he was facing. Otherwise, he was just a normal guy.
The specific issue here for me is with the discontinuity between how Banner has been written
in the MCU and Ruffalo's performance. What I don't buy isn't "Ruffalo could be
a Bruce Banner" - he totally could be - but rather "Ruffalo could be a Bruce Banner whose defining secret is that they're 'always angry'", because I don't believe the latter from his performance, whereas that seemed to be exactly Norton's portrayal.