The Capitol Police (USCP) is charged with protecting the United States Congress. It answers to the Capitol Police Board who are appointed by the legislative branch of the federal government of the United States.
The Capital Police Board consists of three voting members: the Sergeant at Arms of the United States House of Representatives, the Sergeant at Arms of the United States Senate, the Architect of the Capitol, and chief of the Capitol Police who is a non-voting member of the board.
The Sergeant at Arms of the United States House of Representatives is under the direction of the Speaker of the House or other presiding officer, the Sergeant at Arms plays an integral role in maintaining order and decorum in the House chamber.
The Sergeant at Arms and Doorkeeper of the United States Senate and is the protocol officer, executive officer, and highest-ranking federal law enforcement officer of the Senate of the United States.
The Architect of the Capitol is the federal agency responsible for the maintenance, operation, development, and preservation of the United States Capitol Complex.
https://www.politifact.com/factchecks/2021/feb/25/facebook-posts/no-capitol-security-not-only-pelosis-responsibilit/
Capitol security is not solely the responsibility of the House speaker. It is provided by the sergeants-at-arms of the House and Senate, and by the Capitol Police.
The House sergeant-at-arms reports to the House speaker, or Pelosi at the time of the attack. The Senate sergeant-at-arms reports to the Senate majority leader — on Jan. 6, Sen. Mitch McConnell.
News reports indicate that in the days before the attack, House sergeant-at-arms Paul Irving resisted calls from the Capitol Police to bring in the National Guard for extra security at the Capitol because of “optics.” Irving later testified that intelligence reports didn’t show the need for the extra security, not that he rejected it because of optics.
--------
Was Pelosi responsible for the National Guard not being there on January 6th? Maybe not directly but Irving who "is under the direction of the Speaker of the House" didn't want the National Guard present even after the Capital Police asked to bring in the National Guard for extra security.
So as I said, Pelosi did not have the power to make that decision.