Do you think the smallfolk’s support for the Kingswood Brotherhood was a result of Tywin Lannister’s undoing of Aegon V’s reforms?

There’s not a lot of evidence (and fyi, Tywin was finishing the job that Jaehaerys II started, so let’s not let the latter off the hook), but it seems like it:

“If you want their help, you need to make them love you. That was how Arthur Dayne did it, when we rode against the Kingswood Brotherhood. He paid the smallfolk for the food we ate, brought their grievances to King Aerys, expanded the grazing lands around their villages, even won them the right to fell a certain number of trees each year and take a few of the king’s deer during the autumn. The forest folk had looked to Toyne to defend them, but Ser Arthur did more for them than the Brotherhood could ever hope to do, and won them to our side. After that, the rest was easy.”

Dayne essentially won that conflict by giving the common folk enough incremental gains to outweigh the Brotherhood’s redistribution-through-theft.