I accidentally told someone from portland, that people from portland are losers.
Why did you tell them they are losers?
A guy asked me what I wanted to say about Portland, and I didn't realize he was from there
You shouldn't be so careless when saying things to people that you don't know.  But more than that, it's wrong to judge an entire group of people based solely on where they live, since you clearly didn't want to offend any individual from Portland.
It's just how I feel. I feel so many people in Portland are losers.
Well that is your opinion and you should feel free to express that. There is a time and place though to share it, so try to be more sensitive to people who you may be offending.