I know the Might and Magic CRPGs are part of the Dungeons and Dragons universe. Is the Heroes series technically D&D games as well, part of the same universe?

What? No they're not. It'a a completely different universe.

Yes, HoMM shares the universe with M&M.

What the hell...I could've sworn it was. I looked it up, and it's not. Never mind. I'm probably thinking of another game.

The HoMM and M&M series are not part of the D&D universe, no. However, a lot of the game art for the early M&M games was made by Larry Elmore, who also famously did a lot of the cover art for the Dragonlance novels, so possibly that might be why you thought they were linked. (Elmore's dragons have a very distinctive look, for instance.)