She most certainly had seen what Naminé could do. There's very definitely room for improvement overall, but that would have been true of anyone who'd been just starting out.

"Maybe?" she answered with a shrug. "But even most of the fire spells I know don't actually set things alight, most of the time."

There's a different spell for actually setting enemies on fire, and Naminé hasn't actually mentioning having heard about that one. Not, of course, that Aqua had asked for a full list of what she'd been told about, but she'd have almost expected that it would have come up under the circumstances.