I see two aspects that define stupidity in a practical sense.
Stupidity is either the INABILITY to respond to evidence or the UNWILLINGNESS to respond to evidence.
There exist very smart, even brilliant individuals who have the ABILITY to respond to evidence but in some categories will still stubbornly be UNWILLING to do so.
So to avoid stupidity regarding any issue requires both the CAPABILITY as well as the WILLINGNESS to do so.
However, and I cannot emphasize this too much, you can never really BE a "stupid person". Stupidity is a measure of your behavior (thoughts, feelings, actions) and not a self-definition.
Anyone can do stupid things but there is always the possibility for learning to do better.
Identity is not utility. The machine is not its product. You are not what you do.
P.S. There is no such thing as a stupid cat.
P.P.S. CAPS here are for clarity of reading and not meant to imply anything else.