It is only lame duck if he loses an election
“Actually, President Trump will be a ‘lame duck’ if he wins re-election as well.”
The traditional definition of “lame duck” is a person or group who continues to hold office after a successor has been elected but has not yet been inaugurated. In recent years, this definition has been expanded to include anyone who cannot be reelected to an office, e.g. a second-term President.
I prefer the stricter definition, so President Trump, if reelected this year, will indeed become a lame duck, but not until after the election of November, 2024.