The world of full of all types OP.
Bitchy jealous ones, regretful ones, ones who enjoy others success.
I disagree to some extent, that talking about money is taboo. I don't mean present spreadsheets in the pub, but many Europeans aren't as uptight as the UK about money and seem to fair better because people learn from each other and their small societies are much more equal in financial terms. Hence making more people content, or at least not asl bitter.
Everyone has the potential to be jealous, that is entirely their problem but how you talk and divulge information can make a difference.
We are not mortgage free, but wouldn't be jealous of you for being mortgage free. Would you be jealous of "us" for our happy marriage and that our kids have both devoted parents together in the same house? Because if you are, and I really don't expect you to be, there's nothing we can do about that. But we don't compare ourselves and say "oh poor x it must be hard work being a single mum but you know we made the right choice who we had children with and didn't "front load" our child bearing so I guess they just have to live with their choice of father".
You see how easy it is to be judgemental? So I disagree that topics are off limits. But how you approach them and respond to them is what makes you "fake and nasty".
Your post comes across a bit like you judge them for making different financial decisions than you as much as they are judging your mortgage free status.
We could perhaps be looking at retirement now, late 40s, had we not spent our 20s disposable income on wine, travel and song. But oh boy am I glad we did. We will never get our 20s back, being responsibility free, healthy no creaky body and lots of fit young sweaty people to waste endless hours having fun with at festivals, on dancefloors, would I exchange that for a slightly earlier paid off mortgage. Not a chance. So perhaps that is where you can start, even if you don't mean it, even if it's not true, you can be humble to help them feel better and not jealous you can say "yeah but sometime I wonder if I misspent my 20s being too sensible" "I wonder now if I had of gone to Peru when I had the chance would that have been worth another couple of years of mortgage". That sort of thing. It's how you socially make it harder for people to bitch about you, it also makes you more humble and less judgemental.
Perhaps being financially sensible is who you are. We still want to travel maybe that's just who we are. We could be mortgage free, I think, we could get a bigger house I think but we wander onto planes, boats, trains instead. No way is better than the other they are just different routes to our coffins.
If you want to remain 'friends' as 'finding new friends' isn't quite as easy as it sounds sometimes, perhaps point out your sacrifices rather than your gains. Everyone knows you are mortgage free now you might as well own it. But not in a martyr way, in a "I am jealous your younger days way". "I missed out on the holidays way paying a mortgage is boring don't you think" way.
Don't apologise for being you, don't gloat either.