Should a post doc recruit and supervise final year undergrad students when PI is getting older and wants to reduce involvement?
There are two basic problems here.


The interviewing process is too time-consuming.
You are being given a responsibility without the corresponding authority, i.e. the grunt work is being delegated to you, without the decision-making authority being delegated to you along with that.


If you "ran the zoo" (I'm quoting the children's author Dr Seuss), in other words, if this were your project entirely, I think you would organize the application process in a more streamlined way, that would take up much less time.  For example, you might develop a list of questions each applicant should answer, and you might send the questions to the applicants via a google form, so you can easily compare the responses in a spreadsheet.

It could seem to you like a waste of your time to find out interesting things about the applicants in the in-person interviews, for the PI to just ignore all that anyway when making the selection.

You might want to assert yourself politely with your PI:


  Explain to the PI that you want to support her decision to step back from the day to day running of the lab. Let her know that you find this assignment intriguing and helpful for you in your own career growth.  Explain that if you take this assignment on, you would like to have her blessing to handle the whole assignment autonomously, from beginning to end.  Ask if she would like you to submit a final report on the decision(s) you came to.


Project confidence.  (No complaining about how much time it will take you.)  

It will look good on your CV.