Dragon is a specific creature type.
From the Monster Manual:
A monster’s type speaks to its fundamental nature. Certain spells, magic items, class features, and other effects in the game interact in special ways with creatures of a particular type. For example, an arrow of dragon slaying deals extra damage not only to dragons but also other creatures of the dragon type, such as dragon turtles and wyverns.
Dragons are large reptilian creatures of ancient origin and tremendous power. True dragons, including the good metallic dragons and the evil chromatic dragons, are highly intelligent and have innate magic. Also in this category are creatures distantly related to true dragons, but less powerful, less intelligent, and less magical, such as wyverns and pseudodragons.
Since a chimera does not have the Dragon creature type, it is a Monstrosity, it’s breath weapon doesn’t count for Dragon Scale Mail.
It wouldn’t be unreasonable to rule otherwise.
Above is the RAW ruling, but it seems perfectly reasonable for a DM to rule that the Dragon Scale Mail gives advantage against the chimera’s dragon head breath weapon. It fits thematically, for sure.