The four seemingly smartest people have so far been INTX and EXTP.
By "smart", I mean either very knowledgable or very good at getting things done.
Haven't met a dumb or stupid person.
^This. Sure, there are "dumb" and "stupid" people in the world --- comparitively speaking, but MBTI actually has very little to say about intelligence in types - except some descriptions that are poorly worded.
One of the things is that people are not acknowledging one very simple thing. It's reverse relationing ... meaning that they see a smart person, at the same time they have an association of smartness with a particular type
therefore they type that person as the type they associate with intelligence hence reinforcing that stereotype further and saying "well, the smart people I know are such and such type, therefore such and such type are innately intelligent" ... Nope, that's not what this system is designed to help anyone do. At all.
On the flip, people who associate stupidity and type would do the same. Type everyone they deem stupid to be one of the types they already deem stupid.
If typing works and is a good system, then the best way to work through the jumble is to go beyond the most standard and easily accessible descriptions on the top google pages and read the resources on cognitive functions and how they work that don't show up on the first page of a quick and easy google search.
It's taken me 7 years of off and on passing interest in the system to develop an understanding of this system (it is indeed more complex than I gave it credit) and while 6 years ago, I was considered well read on the subject by some here (I considered myself one too), I only recently realized that even then I barely got into the system as in-depth as I should / could have.