Nah, that’s simply not true - most drake mounts are unarmored, and most dragons in the game are as well. Even then, the armoring (barding) on the dragons is extremely limited. Further, a dragon in armor is still very clearly a dragon, but a draconic humanoid in armor has far less identity.
Dracthyr need to lean into the “dragon” part of their identity, and that means showcasing more of it, not less. That means relying on their natural scales for protection and leaving them on display.
As to the Mechagnome argument… while this may be a “core race” they’re clearly not treating it like one. The Dracthyr model is a recycled rigging and skeleton, making them, in technical terms, an allied race, not a core one. Maybe this’ll be changed, but the point is, the idea of a “core race” is so broad and varied that taking it to explicitly mean, “they can transmog all the things” is an interpretation, not an explicit implication or rule.
What is “core”? Does it mean a capital? Major significant characters? Fundamentally unique assets? An expansion of clear focus and story…? Who knows?
Most races don’t get a capital, core or otherwise. Allied races CAN have capitals. Core races aren’t necessarily important to the expansion they debut in. Allied races means that core races often have to share assets, even if they were “theirs” first.
The term “core” race can mean… basically whatever Blizzard wants it to mean at any given time. The most you can say about it is it isn’t an “allied” race, but that term itself is fuzzy. Does it mean “derived from another race?” No, that would exclude the Vulpera. Does it mean “variant recycled assets?” No, the Kul’tirans are wholly unique. It’s a broad term that has no hard and fast definition, the only exception being the extent to how significant it is.
And even then, I’d argue the Goblins and Worgen weren’t particularly important to Cataclysm’s main story, so what then? “Core” just seems to mean… not introduced as part of a broad swath of similar-ish scale options, and that’s about it.
So… yeah, that term doesn’t really say anything about what you can expect of the Dracthyr. It’s a blurry generalization, nothing more.