I know of no reason or evidence that mugwort (Artemisia vulgaris) would induce mania. I suspect they are confusing Mugwort with the related Wormwood (Astemisia absinthifolium), which people believed when made into the liquor Absinth could cause “insanity”, it doesn’t by the way. There are not many herbs known to trigger mania, a few that can are Rhodiola and Mimosa bark (Albizia julibrissan).