Oh bless her, I can completely understand why this is bothering her OP. There's nothing wrong with her or her situaton but it must be so hard for her to see this when she's the one dealing with it. At 21 she's still very young, although she won't feel it, and many more people her age are in the same boat as her than she realises!
A couple of people in my friend group alone didn't kiss anyone until the ages of 23/24, I have a friend in her late 20s who is a virgin, everyone experiences life and relationships as different paces and she won't be stunted or left behind for taking her time. It's important she realises that she shouldn't rush anything with someone she may not be entirely into, just to not be "a 21 year old who hasn't kissed anyone". Kissing and relationships in general aren't like a boat that she will miss if she doesn't jump on quick enough, watching her friends sail away without her, it's more like a train station where a train comes ever 5 minutes, if you don't catch one you get the next one. She will still get to the same destination, just at a different time, nothing is lost because of this.
Also, (if she is the sort of person who won't take this the wrong way) you can tell her in the absolute nicest way possible and with no bad intention, that literally no one cares that she hasn't kissed someone at 21. No one is judging her for not having kissed someone. No one thinks she's lame or slacking, no one thinks badly of her, she is more concerned about this than anyone else will be and she needs to relax as relations will come in their own time, she just needs to ride the wave and she will find the right person when they come along.
The fact she is preparing to leave uni which is a huge milestone, her age now starts with a "2" rather than a "1" and she's no longer a teenager all mean she probably feels like she's running into full-blown adult life but you need to try and gently remind her, in the least patronising way possible, that she is still only 21. She's young. She's got decades of loving, romance, kissing, hugging and dating ahead of her if she wants it but in the grand scheme of things, being a year or two behind your friends and siblings means very little in the overall pathway of her life. It's her life, not to be compared to anyone else's 🙂