Yes. As they have done recently with Measure M (transportation) and Measure H (homelessness), the County or other regional agency could place a sales tax measure on the ballot to increase the sales tax in the City. That money would go to the County or other regional agency and potentially none of it spent to directly benefit Diamond Bar. A County sales tax measure could be imposed in Diamond Bar even if a majority of Diamond Bar voters voted against it.
Generally, state law caps sales tax rates at 10.25 percent in the City. The current City rate is 9.5 percent. If Diamond Bar voters were to enact the local 0.75 percent sales tax increase, the local rate would be capped and no further local sales tax increases could be applied to Diamond Bar.