Is a filter a good way to accomplish a dynamic class in angular?
Fun question, I think this is okay.

I would name klass -> class and I would consider building a map for the values:

var classMap = [
  { from : 3 , to : 5       , class : 'alert-success' },
  { from : 5 , to : 10      , class : 'alert-warning' },
  { from : 10, to : Infinity, class : 'alert-danger'  }
];