The very few people I see here daily (in Bangkok) wearing masks probably wear them for all sorts of reasons with air quality being at the bottom of the list, is my guess. At the top of the list: no time for the full make-up treatment? Don the mask.
That said, there is an annual burning season - end of dry season heading into rainy season - where owing to both local and regional burning the air pollution is off the charts. Chiang Mai is often severely impacted (AQI: 150-200), Bangkok less so. February, March and April are the worst.
When (approximate month) will you be in Chiang Mai? At the peak of burning season a mask is of little help to the eyes.
While available here, products like the Nexcare N95 "Haze & Pollution" masks may be in short supply, so maybe bring your own?