the simple and brief answer is ,
ITS IMPORTANT TO BE SMART.
Perhaps , you relate the word "smart" to "cunning or shrewd"
NO , smartness is just finding a solution more effectively or quicky , and its just a function of brain .
OVERSMART literally means nothing. I have heard this word since I was in school myself, but it has no sense.
People either use it to mean that someone is SHREWD , or maybe they are dumb themselves
so , BE SMART
Keep POSITIVE intentions