Is Chiang Mai culture all about expats?

I'm not really sure what you mean by "Chiang Mai Culture."

It would be more than a little self-important to assume that the whole culture of the place is based around ex-pats, wouldn't you say? If you decide to hang around ex-pat haunts with other ex-pats, of course the culture in those bars or condo buildings are going to be ex-pat orientated, but Chiang Mai is still in Thailand and the "culture" is of course Thai.

I lived in Chiang Mai for 3 months, by the way.

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Simple answer is it leans towards Thai culture, anywhere in the north of Thailand does. Well the whole of Thailand does, but the north is less influenced by western ideology. But you will never get too far in a Thai city from the western influence; you're not going to avoid it.
If you want Thai, as much as you can handle, move 20 or 30 Km out of Chiangmai and live in a village. Your right to respect Thai culture and I can appreciate anyone wishing to immerse themselves in this very strong family based lifestyle. But be warned it's not for the faint hearted, I feel Chiangmai city is a good half way house for some one looking to put roots down, to start with anyway. Good luck!