An attraction's classification can certainly evolve with time. Revenge of the Mummy was certainly marketed as an d-ticket despite its relatively low budget. Because it was the park's only real rollercoaster at the time, I think it's safe to say it debuted as an e-ticket, too.
Now? It's a gray area, I'd say. I've always liked The Unofficial Guide's categories, which arguably convey a bit more nuance: Minor attraction, major attraction, headliner, super headliner. I'm not actually sure what the latest Guide classifies Mummy as, but I'd put it under "headliner." If we want to stick to the numbered letters, I think it slips to D with the opening of Fast & Furious.