The confusion about whether or not it is "as big an issue as some people make it out to people" is because influential members in the various forums and Facebook groups have a vested interest in down-playing it, their business in some way benefits from having a steady stream of nomad newbies arriving throughout the year. They try to shout down any mention of it, and will outright lie when someone asks a question such as yours.
Do real research on particulate pollution and look at the stats. The haze is genuinely bad for your health long before it becomes as visually obvious as it does in those months. A year in which the smoke season is considered to be "not so bad" can actually do more damage, because people are inclined to spend more time outdoors.
My opinion is that, although Chiang Mai is relatively cheap, no-one should move here unless they can also afford to spend those two months elsewhere in Thailand or South East Asia. Most Westerners on tourist visas have to spend stretches of time away from Thailand anyway, so, I treat the smoke season as an opportunity to spend time in Vietnam or The Philippines.