Making the Node Labels always visibke is as easy as setting labelThreshold to 1

s = new sigma({
  graph: g,
   settings: {
    minNodeSize: 4,
    maxNodeSize: 2,
    minEdgeSize: 1,
    maxEdgeSize: 1,
    labelThreshold: 1
  }
});

https://github.com/jacomyal/sigma.js/wiki/Settings

(thanks to:
https://stackoverflow.com/questions/22434121/how-to-inactive-sigma-js-node-hover)