The Mothers would certainly exist in all the universes where the Mantles exist, but the Courts and the Mantles are at most a couple of thousand years old, so this would only be a percentage of the Multiverse. Thus whilst there may be one of them, they are the Mothers over that percentage. Where the Fae and the Courts do not exist they have no reason to be, and there may be other beings occupying a similar position over a other local groups of universes. Conversely if their local group of universes succeeds in defeating the Outsiders it allows further branching of these universes and an expansion of their being. They are very pro Harry succeeding, even if they have a back up champion on another timeline where they exist because it means they start the next cycle at a much higher percentage of universes where they exist, more opportunity for branching of their universes, and their own personal growth.
Uriel has been described as being able to snuff out galaxies, his power runs through the multiverse, throughout each universe, including timelines where the solar system or Earth never coalesced, life never existed on Earth, humanity never evolved etc but life evolved on other worlds giving rise to alien champions totally apart from Earth all being watched and shepherded by Uriel and the other Archangels in the battle against the Outsiders. Earth is this local group ofuniverses battleground.
The White God and the Archangels are spread all over Creation from, well it’s creation. The Mothers would only exist over their little part of creation.