Some characters(mostly princess peach) wear some somewhat skimpy outfits, though there is nothing really shown.

A scene where Peach is being held by the straps on her tank top. We can see into the tank top for a brief second.

At the end Peach leans into Mario and seductively talks to him. It is ended abruptly though.

The overall tone of the series is dark. A small band of rebels known as the smash club fights against a large empire. In the first episode shy guys kill guards. A character sacrifices himself and it is very emotional. The sacrifice is not shown or even heard but implied. Crying takes place and the scene gets pretty intense. Later in the series a bad guy talks about how the dead character was "begging for death" before it happened to mess with a protagonist.