Seeing as she spends most of her time at Youkai mountain, she probably got used to them after a while. It's hard to see youkai as particularly strange or interesting when you practically live with them.
I mean, let's see it as Sanae's perspective.
So first, she's in our world. Youkais are pretty much old legends, monsters that are known to be fairy tales. She's also known as being a goddess (at least, to the humans) and her life is dedicated to her goddesses.
Suddenly, Gensokyo. Youkais are real, they're everywhere, welcome Sanae, this is the ''real'' world. She doesn't deal with it very well, her first reaction being ''Let's do what Kanako says'', hence the very pretentious attitude, in my opinion. In Touhou 12, she's a bit more ''Herself'' but she's told my Kanako/Suwako; ''Kill youkais, it's your duty!'' or ''find treasures!'' She follows said orders dutifully.
Finally, Touhou 13 where she's pretty much used to the whole ''Youkais are a majority, humans are a minority'' mindset (complete opposite of what she originally lived in), hence the ''I don't really care anymore'' attitude. She doesn't have any clear orders from Kanako/Suwako, so she just investigates it for the hell of it. (Like everyone else)
That's just my opinion though. I still can't explain the sudden love for food though :/