Im pretty sure it's impossible to hook the mixin, so you need to hook the frame. Maybe this will work (untested)
Lua Code:
local function ShowBountyTooltip(self, bountyIndex)
print(bountyIndex)
end
for _,frame in pairs({WorldMapFrame:GetChildren()}) do
if frame.BountyName then
hooksecurefunc(frame, "ShowBountyTooltip", ShowBountyTooltip)
end
end