I think kisses is one area that may fall into Good or Bad. There are not really many degrees. This is my experience when talking to guys and girls. I haven't heard of scales when it comes down to this ("He is an 8!").
"That guy is a regular kisser", "She is not that bad at kissing", is not something I hear. For my experience is more about "He/She is bad/okay/good/great"
Bad/Okay is BAD.
Good/Great is GOOD.
Then you must aim for great, of course. I really think is not that difficult. In my experience I think I have always been a natural good kisser as well as in sex (I can last like a fucking pornstar. I do actually practiced kegel exercise and techniques when I was teenager because I wanted to endure and be great in bed).
If we are talking about lip kissing, here are some of my recomendations:
• Don't go for tongue-kisses at first: Build excitement. She might even want to get more at one point. It's great if you try to keep it flowing slowly so she gets really turned on to the point where she wants to keep kissing you (make her crave your kisses) like crazy. It will also help escalation.
• Play with kissing styles: So, you give her a square-kiss (like both your lips on top of her), then you kiss her bottom lips, the upper lips, then some lip-tongue-brush (from bottom lip to top lip while your lips are slightly touching) ending with a kiss (maybe more on the upper lip), a little bite on her lower lip (don't thing upper bitting is sexy. I think they don't even like it that much), then some tongue action.
• Don't force her: It means not to force your kissing style. Build her desire with your kissing. There will be a moment (depending on the girl) when she will clearly start doing this and you will know what she likes. If she does it, she likes it back (that's why when learning oral sex I would recommend girl on girl scenes. Although male pornstars know what they are doing, sure). Some girls like a lot of tongue when kissing, others lightly touch. I usually kiss the tongue when they are putting it in my mouth (I don't know if girls like this or not but when they have imitated my move, I like it

. Doing this will help you know what she likes.
• Start making out: This means kissing her with the mouth wide open. This will be when she is really horny, excited about kissing you. You will know because, now, she will start being really forward with the kissing because her excitement level will be of the roof (probably she will be making sounds like moaning)
• Keep going back to other styles: Don't get stuck on this open mouth all the time.
• Give some break: I started using it because I got tired. Eventually it became part of my style to stop and resume. Give her some intense look, look at her lips, then make eye contact again and compliment her on her lips, kissing, something relevant.
• Use other type of touching: Is not related with the kiss, but is good to give her more stimulus. So, while kissing her, slowly brush your fingertips on her skin (arms, thighs, side of her body, neck, etc) in different parts of her body. Don't jump from place to place like crazy. Make it flow in a natural way.
This is the advice I can give you. And, don't be afraid to practice with the top of your closed fist (I did it when I was younger and I hadn't kissed no one).
Cheers!