Trump's approvals are not at 54% in Missouri.
He got 46% of the vote nationally and 56% in Missouri in 2016. He's now at 44% approval nationally. Are you just ignoring all the recent polls and pretending he's still at 40% approval or something?
2016 results are a universe of their own. Your statement would be more relevant if Trump won completely on his own merits in 2016, as opposed to a significant chunk of the vote not only voting
against Clinton, but many even voting 3rd party.
In this sense, it's perfectly plausible for Trump to be slightly more popular in 2020 yet win Missouri by significantly less, or even lose it entirely, so long as his Democratic opponent is semi-popular and not a total dud. It's also shouldn't be surprising for MO Republicans to do worse this cycle despite Trump being more popular than he was in 2016, for the aforementioned reasons.