I see TNG with mostly 2D characters where the Federation and its ideals are the main driving force of the plots. When they deviate from that is when you get bad episodes (cough Sub Rosa cough). The characters had to shed some of their depth and become idealized for message to shine through.
On DS9, you have a gritty view of a frontier without the influence of the Federation. The evolution of the characters and how they react to the changing reality around them is the center stage, and for that you need 3D, flawed characters to build development arcs upon.
Then on DSC you have perfect 2D characters in a corrupt world and the show is about Michael Burnham but she’s also perfect and I can’t see what message they’re trying to send.