Being smart in one field doesn't translate to being smart in another field. For example, Steve Jobs making medical decisions, doctors making investment decisions, bankers making tech decisions.
Yes but the topic is "Is Elon Musk stupid?"
I would wager that anybody widely regarded as a genius has a decent sized rap sheet of stupid crap they pulled. So "made stupid impulsive buying decision" is great evidence for impulsivity and hubris, not "stupid."