#1, that was never her title, that's not a thing. #2, she was not tasked with doing ANYTHING directly related to border security, as in trying to actively stop the migrants who are already here/on their way from crossing the border. She was assigned the relatively narrow task of examining and helping to address instability and economic crises in El Salvador, Guetamala, and Honduras. This is a narrowly focused LONG-TERM project, that perhaps eventually we can help stabilize these countries to the point that so many people aren't made so desperate that they need to flee. EVENTUALLY. #3, she didn't even do a bad job! She did make some gains in getting the private sector to make more investments in the region. But it will take a long time and a lot more work from a variety of people and agencies to really see the work pay off.
Personally, I don't think Biden should have assigned this duty to her. The border is political lightning rod and Kamala's ability to go too far outside of DC for any period of time was hampered by needing to break ties in the Senate. But still. This is deliberately disengenuous from the people making the argument and it's getting on my last nerve.