Identity
Dihydroquercetin — more precisely taxifolin — is a flavanonol, the dihydro (saturated) form of the well-known flavonol quercetin. It is extracted mainly from larch (notably Siberian larch) and other conifers. In cosmetics it is used under the INCI name Taxifolin as a plant-flavonoid antioxidant. (Like its flavonoid cousins, it is also sold as an ingestible supplement — outside this register's topical scope.)
Mechanism (as proposed)
Taxifolin is a strong in-vitro antioxidant and radical scavenger, and is reported to have capillary-stabilising (microcirculation-protective) activity — the basis of some "anti-redness" and "vascular support" cosmetic positioning. The antioxidant chemistry is genuine. As with the other plant flavonoids in this register, the honest limits are delivery-side: penetration through intact skin and stability in formulation determine how much of the lab activity actually reaches living tissue.
Reading the evidence honestly
The missing piece is the same as for myricetin and mangiferin: little controlled human topical data for any skin outcome. The cosmetic case rests on in-vitro antioxidant and capillary-related results, and antioxidant potency in a dish translates unreliably to visible skin benefit — hence the D grade despite a capable laboratory profile. The larger taxifolin evidence base concerns ingestible use, which this topical register does not cover.