Mugwort has one genuinely early European attestation, and it is worth separating from the mass of later material.
The Nine Herbs Charm is an Old English metrical charm preserved in the tenth-century medical compilation known as the Lacnunga. It names nine plants and addresses them directly, and mugwort is the first: Gemyne du, mugwyrt, hwaet pu ameldodest: remember, mugwort, what you made known. The charm calls it una, the oldest of herbs. That is a real Anglo-Saxon text giving mugwort priority among healing plants, and it is the source of the epithet mother of herbs that later writers repeat without citation.
Culpeper in 1653 assigns mugwort to Venus, and his ground is the standard one in his scheme: he uses it for complaints of the womb and the menstrual cycle, and in the Complete Herbal that class of use puts a plant under Venus almost automatically. The reasoning is systematic rather than observational. It is worth noticing that Culpeper's Venus category is doing a lot of work across many unrelated plants, which is a general weakness of the method.
Later occult compilers frequently move mugwort to the Moon, on the strength of its reputation for divination and dreaming. That reassignment is modern and it is not an emendation of Culpeper; it is a different tradition reaching a different answer for different reasons, and sources that print the Moon attribution as though it were the herbal's are simply in error.
The charm itself should not be over-read in the other direction either. It is a Christianised text, it invokes Christ and the nine glories, and it sits in a manuscript alongside Latin liturgical material, so it is not a window onto pre-Christian Germanic religion, whatever it may preserve. What it does establish, securely, is that mugwort held first place in a list of healing plants in tenth-century England, and that is a genuinely early attestation of the plant's standing.