A mascot existing wouldn't inspire towns to invest resources in developing their own. Sure, they mah have had some hopes that it would lead to similar merchandizing sales or tourism growth, but that dream likely died quickly.
A cute character regularly reminding people of their home to take advantage of this tax makes more sense to me.
Even though the furusato nozei system was originally created around the same time as local mascots started to become popular in 2008, there wasn't competition from municipalities to try to attract donations until around 2016.
Mascot characters like Kumamon, which were created after 2008 but before the furusato nozei system really started attracting attention, were specifically created to attract tourism, not to attract donations.
So even though it might seem that the timing would match up, it's not really true that there is a connection.
Furthermore, when municipalities started trying to compete for donations, they didn't use mascots, they just started bribing people with return gifts. Maybe in theory municipalities could have used mascots to try to attract donations, but in reality they don't seem to have done that.
I think you’re underestimating what a phenomenon Hikonyan was, most tourist attractions would be happy if their character made them an extra 1% of what Hikonyan made for Hikone Castle
