I also think romances are most popular with women, and online fandom participation is more popular with women. So it does not surprise me that Astarion fans eclipse the fans of other characters in fandom spaces. No hate on that since I am part of that demographic. But I wouldn't use that as a metric of which character is the best written since I've seen many of these same fans boot or kill Shadowheart because she overrides Astarion's scenes. Give Astarion a chance by all means, but do the same to the others lol.
I don't think so. Bioware have a history of releasing stats on their romances and players have always tended towards their straight dudebros. Astarion is eclipsing Lae'zel and Shadowheart because Lae'zel and Shadowheart are disasters of character writing. The fact that no-one is romancing Shadowheart in spite of the fact she's the only human looking option available for the largest section of the playerbase is a sign of quite what a mess she is.