It's not even the best fantasy girl anime of the season but, the anime's done by KyoAni, so it's popular.
I doubt that it's just because of KyoAni. Phantom World seems to be treated as relatively "meh". And at least here, it only has roughly double the amount of fanart Dragon Maid has. Considering that it's over since nearly a year, while Dragon Maid isn't even quite half finished...
Also, among viewers Demi-chan is the most popular show this season. Amount of fanart does not always equal viewer popularity.