The mayor of DC oversees the police force that's responsible for policing off the Capitol grounds, in the rest of DC that's not federal land. They're not the ones who would be asking for reinforcements to help secure the Capitol, they're one of the potential sources of reinforcements for the agency that is responsible for protecting the Capitol.
The mayor's request was basically that the Feds not unilaterally start escalating and militarizing police efforts in her jurisdiction without her involvement. In hindsight, that does seem to have been the correct strategy for her police department, because all the violence seems to have been concentrated within the jurisdiction of a different police department.
Sure, but there's a difference between coming with riot gear and attacking protestors, and posting 10 cops to guard all of congress and then have them walk away when protestors show up leaving the doors wide open.
They could have easily guarded the building itself. Close the doors and post 30 guards at each entrance?
Interestingly, everything I read today said that they did request assistance. A day or two ago. It just took until the events unfolded for anything to happen. A very quick Google search turned up: https://abcnews.go.com/Politics/wireStory/urging-calm-dc-may...
They could have guarded the building, though, even if they don't want the agents to engage outside of the building. Guarding the entrance should be feasible. If it isn't, Congress should not have meetings there.
The mayor's request was basically that the Feds not unilaterally start escalating and militarizing police efforts in her jurisdiction without her involvement. In hindsight, that does seem to have been the correct strategy for her police department, because all the violence seems to have been concentrated within the jurisdiction of a different police department.