This is a result of the character model facing forward along the wrong axis. Unreal convention (and the setup used for Mannequin and MetaHuman characters) is to have the character facing forward along the +Y axis.
It can easily be fixed by adjusting the Import Rotation value in the Import Settings of the Mixamo character, it should usually be set to the default value of 0,0,0. This option can be set in the FBX Import Options when first importing a model, or it can be changed in the Skeletal Mesh asset after importing. Note: After changing the value in the Skeletal Mesh asset you have to click the 'Reimport Base Mesh' button to apply the change.