Yue wasn’t born a spirit, and she vanished basically as soon as she took on the Moon Spirit.

The original Moon Spirit was a big ol’ fish.

Hey come on, she wasn’t a spirit when he met her!

Hei Bai, Wan Shi Tong, Koh - those are spirits that Aang has met. Now if you ask me, Hei Bai is beautiful in his panda bear form, but the monstrous form he took when Aang met him? Not pleasant to look at.

To add to what other comments have said, wasn’t Aang fighting the Fire Nation when this was going on?

So even if we want to make the argument that Yue is a spirit, he didn’t see her as a spirit.