It’s not a dumb question at all, and there is no “agreed upon” definition.
For me the most important characteristics of a “Mainstream Distribution” would be the size of their maintainer team - though that is also inaccurate if we are talking about distributions that are built on top of other distributions - as in your example.
Another indication is to check who is sponsoring a distribution’s development. If there are plenty of commercial sponsors, then chances are that the distribution is well maintained. Similarly, if the distribution is created by a commercial company (Intel, Canonical, RedHat,…), as those companies also have an interest in keeping their product in a good state.
Age of the distribution might be another indicator. If a distribution has been around for a long time, chances are it isn’t bad either.
However, I am lazy and would not actually check any of this by hand. Instead, the thing I would actually do is to just go to distrowatch.com/dwres.php?resource=major and read through their list. 😉