I guess so. What a user submits to relays is intended to be public, so maybe there would be data there you could use. I'd be okay with them knowing my country of origin. I wouldn't want them to partner with anyone else tracking me elsewhere on the internet though. Once you start displaying stuff about me that Google knows, like a location I was recently at, I really think you are cutting into what is enjoyable and charming about Nostr. There are reasons why, because of the demographics attracted to early Nostr use, such ads may be considered more intrusive than the average user may consider them.