Perceived approachability usually says more about the prospective approacher than the prospective approach-ee.
Almost all women respond well to genuineness and casual confidence. They may of may not prove interested in what's being offered (which of course depends in part on what it is that's actually being offered), but they'll usually be friendly - they'll usually seem approachable - when someone demonstrates those characteristics.
In other words, almost all women are approachable to the extent that a given guy:
(1) is himself,
and
(2) being himself means being something other than
(a) an arrogant ####### who thinks he's entitled to whatever he wants
or
(b) an insecure loser who doesn't think he deserves anything.
More or less, it's about that simple.