It should be, the only known exception is Andromeda. The impression I got from 1x03, To Loose the Fateful Lightning, and 1x12, The Mathematics of Tears, is that Ship's Avatars are unusual, and only created when necessary. Dylan didn't even think about the Pax Magellanic's avatar until later on in the episode.
To quote the Andromeda wikia's page on Rommie:
Early on in the series, it is stated that avatars were created to
allow the crew to relate to their ship on a more human level, and vice
So, Andromeda would have been able to relate to the crew well enough that a Ship's Avatar wasn't necessary during the days of the old Commonwealth.
Given that Rommie is also capable of overpowering Andromeda (Seen in several episodes, both Rommie and Doyle were able to do things to the ship that the core AI didn't want them to do), it is also possible that a ship's avatar was capable of accidentally restoring backups, just like Harper did in 1x22, It's Hour Come Round at Last. If that were so, the Commonwealth likely decided not to give Andromeda a new ships avatar after she limped back from the Magog galaxy, to help keep Andromeda from going insane again.
Finally, Rommie was noticeably different from the Ship's Avatars seen on all other Commonwealth vessels. In most of them, such as the Pax Magellanic and the Achilles, the Ship's Avatar was an extension of the ship, part of the same AI. Rommie, however, was an independent entity that was simply linked to the ship's AI - in part, likely because Harper improvised when he built her, the most obvious example being that he borrowed some of Beka's hair-nanobots to give Rommie the same ability.