From Populous Wiki

Noobs are people that are new or acting like there new because they ar not as good in Populous compared to the more experienced players.

Some people call people noobs because they rush, camp or do stuff that enemies do not like. Some people call noobs as abuse to people maybe if they are winning and immature or do not like their style.

This is not recommended, the nicest term for someone that is actually asking if they are a noob is asking if they are new. But there are no rules in Populous except from Cheating and Resyncing so noobs does not mean that they are breaking rules or playing by unliked styles.