It means that an intelligent person who does stupid things is still stupid. You are what you do.

Another variation is "beauty is as beauty does" or "ugly is as ugly does".
"are you stupid or something?"
"stupid is as stupid does"
by chocolate fingers August 9, 2006
a catch-all term that basically means that stupid people are gonna keep on doing stupid things because that's what they are, period. The term was popularized in the 1994 Tom Hanks 3-hour blockbuster movie Forrest Gump.
1. Look at several trends, fads and fashions from the past number of years. Many things got dumber and dumber and in this 21st Century many stupid people did evil and stupid things and a lot of people fell for all the lies and B.S. Stupid is as stupid does. Abe Lincoln said you can't fool all the people all of the time. I hope to God he was right.

2. ...
"If God gives you nothin' but lemons, then you make some lemonade"
"The early bird catches the fuckin' worm,
Rome wasn't built in a day"

"Now life's like a box of chocolates,
You never know what you're going to get"
"Stupid is as stupid does and all the rest of that shit"

Come'on pretty baby, call my bluff
'Cause for you my best was never good enough

Bruce Springsteen
by I Saw U2 Live Twice June 15, 2009