Mirror text on 8 way 7219
The MD_MAX72xx library has a transform function. 

Depending on how you want to mirror the text (along which axis), you give that function the MD_MAX72XX::TFLR or MD_MAX72XX::TFUD argument.

Though I'm guessing that you might actually want to rotate the text 180deg. In that case use transform(MD_MAX72XX::TRC).