Most of the traffic in mobile operator networks I'm aware of is IPv6 to IPv6, for very obvious reason - people watching youtube, facebook et al, and they are IPv6 enabled for over 10+ years. Of course one need to provide IPv4 reach-ability as well, details are varies from network to network, 464XLAT is being a popular one.
The big reason mobile networks use IPv6 is that they are new and don't have the IPv4 allocations of legacy providers. They could have gone CGNAT, but IPv6 with 464XLAT is better. Mobile networks are easier to use IPv6 than home networks since there is no customer gateway to worry about and 464XLAT software is included on device.
It's "popular" in that it's widely deployed, but I think it's fairly unpopular in that people really don't enjoy operating it. When I worked for a high traffic website, I was told people would occasionally reach out like "hey can we just ipv6-peer with y'all so our users don't have to talk to you over our cgnat. it's putting a ton of load on the cgnat gateway".
No one who operates a CGNAT enjoys it or wants to operate a CGNAT. I'd wager that most mobile network traffic is native IPv6 to IPv6 because all of the super high volume FAANG et al websites are operating IPv6.
CGNAT is a set of hacks on top of another set of hacks. You can keep stacking NAT on top of NAT for near-infinite amount of IP addresses, but what you can't do is you can't multiply the number of ports.
Before I switched ISPs, my home ISP was using CGNAT with no IPv6. It was trivial to completely screw up the connectivity for myself (and probably a lot of other users) by simply running a BitTorrent client with connection limit disabled.
