A looker is someone people find good-looking, and in real use it is usually a woman: she’s a looker, quite a looker. It is informal, a little old-fashioned, and it treats the person as something to look at, so it can sound cheeky or dated; many speakers now prefer she’s beautiful or she’s striking. You may still hear it in chat, in older films, and in the set phrase a good looker. It does not mean a person whose job is to look (that would be a watcher, an onlooker, a lookout). The -er here is “someone you look at”, not “someone who looks”.