no, it's nominative with article, so weak declension, it's an -e, if it was indefinite or without article then it would be strong adjective declension -er , ein guter Mensch, guter Mensch
Mensch throws me a little because, growing up with people that sprinkle in a bit of Yiddish, mensch is used to refer only to a good, reliable person, not a person in general.
It's an adjective, so it doesn't have a gender. "Der" is referring to "Mensch."
Because this is in the nominative case (and we have the direct article). "Den guten Mensch" wold be the accusative.