I don't know how low it can go, but does it really matter? It's low enough.
For some reason the article doesn't give the headline result, which is that Trump's approval is at 31% (pdf), a new low for that specific poll (AP-NORC).
Strong approve is only 15%.
I don't think I ever noticed before that even at his peak popularity, his strong approval (in this poll, at least) topped out at 25%.
That's your MAGAs.