Stockwell is not the most salubrious part of London, but it's okay. I'm not a big fan of south London, but yes, there are lots of Portugese cafes and restaurants. To be honest, I wouldn't recommend staying around there on your own late at night if you are not a confident traveller and don't know the area...I can see how it could be a bit intimidating, as parts are a bit rough.
Vicky