Sounds as though she has a pattern, whereby you are her social/conversational tool - she makes jokes about you to make people laugh. Old friends are used to it, probay don't even 'see' it. They may even think it's your 'routine': you're the 'straight guy' in your paired comedy routine. If they think about it at all, they probably dismiss it, assuming you're OK about it.
Patterns like that can get quite set in friendships that were made when you were young and 'finding yourself'. Thing is, you're older, you've changed, she hasn't. It grates, and newer friend are baffled/horrified.
It IS cruel, and it is immature.
A poster up thread talked about 're-setting' the relationship. That is what needs to happen.
You can either talk to her about it. Low-key setting, talk generally about how people change, talk generally about patterns between people and how they change/need to change as we get older, draw analogies from television (or something), and hope she makes the link/lead her to it.
Or you could have a more direct conversation.
Or you could bring it up with a mutual friend and set in motion a kind of mediation. Often, that works well: discuss with mutual friend how this makes you feel (as you have here) and just wait. Usually, conversation within a friendship group will work like a machine that runs self-resetting checks, and the conversatin(s) will work to adjust the group dynamic.
If none of that works - and it might not (she may have too much invested in this pattern; she may not be flexible enough to change; your group dynamic may not be strong enough to facilitate such a change,) then you need to disengage from her.
Avoid a show-down - it will make group meet-ups tough - just avoid her. Don't sit next to her. Don't meet up with her one-to-one. You can try distancing conversational tactics too, of increasing levels of bluntness. Start off with mild ones (abrupt conversation shifts) and just practice to see were your comfort level is. Regard it as practise for you in dealing with awkward people.
It's not a good situation to have to deal with but it is part and parcel of having long term friendships that have to accommodate change.
It does sound to me, though, as though you are - in fact - moving on. Although the situation is making you feel like the weaker part of this dynamic (for obvious reasons), in fact I suspect this is arising/ becoming visible to you precisely because you are growing and developing an inner strength and self belief. That is a great thing to do. Far better to change for the better as life moves on than to cling defensively to past selves and old, negative behaviours.