Looking at the pure definition of stupid, as per the dictionary, we have...
1.
lacking ordinary quickness and keenness of mind; dull.
2.
characterized by or proceeding from mental dullness; foolish; senseless: a stupid question.
3.
tediously dull, especially due to lack of meaning or sense; inane; pointless: a stupid party.
4.
annoying or irritating; troublesome: Turn off that stupid radio.
5.
in a state of stupor; stupefied: stupid from fatigue.
Most people are referring to definitions 1 or 2. The problem is, 2 is subjective, and thus it has expanded to fill a whole lot of space.
People who lack the ability to think quickly and at least around an average level of intelligence, do exist. This is not the average. Chances are, you will not meet many of these people in your lifetime. This is where your "mentally slow" or so forth folk come in. It is a neurological incapability to think on a level of intelligence that is on par with most humans of their age, or some such.
Now, we get to definition 2: foolish or senseless. What is foolish to me, may not be foolish to you. What is senseless to you may not be senseless to me. But, most humans want to think they're right when they have an opinion. This means that the word "stupid" has been used to refer to basically anyone that does something you think is silly, ridiculous, or foolish, meaning that the person may not be *objectively* stupid (definition 1) but can still be called stupid (definition 2). I suspect Definition 2 is a more modern definition created by the expansion of the word to be used as an insult rather than a simple descriptor. An even more expanded form is Definition 4, which basically can be used for anything, so the word sort of loses its meaning around that point.
Thus...yes, stupid people are everywhere, if you choose to think that anyone who does something that doesn't meet your arbitrarily set standard for "intelligence", is stupid. You get to choose how many stupid people you meet, and it's not by hiding in your room.