In some places graffiti is legal and even encouraged. Some places grant certain legal protections to graffiti art and graffiti artists. You may not like it but that’s just how the cookie crumbles. That said, random vandalism is not considered eligible. The criteria clarification collection explains it in this post:
https://community.wayfarer.nianticlabs.com/t/unique-art/
As for most Mediterranean frescos being on single family homes, I haven’t had this experience. I’ve found them on a variety of buildings ranging from condominiums to churches to dentists offices to cafes. I live in a major city though, so our experiences will undoubtedly vary. Of course there are some on private single family homes, but those ones are not considered valid Wayspots. It doesn’t matter if the intention is for the public to admire it. I could put a statue on my mother’s fence for everyone to enjoy and that wouldn’t make it eligible because it’s on the property of a private single family residence. Likewise a mural on a single family residence isn’t considered eligible either, even though it would otherwise. The single family residential rule trumps other questions of eligibility.
It comes down to whether or not the art in question is on a single family residential property or not. If you spot one on a multi-family building or a church or something else that isn’t a single family home, nominate it, it could be a great Wayspot. If you’re not sure, do some digging. I had one approved a couple weeks ago that was on a deconsecrated religious building that’s been converted into apartments. Part of the Wayfarer experience is to discover new things about your community as you explore. Not everything discovered while Wayfaring will be eligible, not everything eligible will be approved, and not everything approved will show up in every game. That’s part of the process.
If you’d like some help with finding potential Wayspots in your area, folks on this forum are glad to help, myself included. However most of the users here are simply fellow users/players and we don’t make the rules for which Wayspots are eligible and which aren’t. We are simply explaining what the rules are and trying to help each other.