Each character is at fixed 8x5 pixels with gap in between each characters. The ROM library of characters can't be rotated and IIRC only has room in RAM for 8 custom characters so even if you designed custom font that is rotated 90' it would be 5 pixels high and 8 wide for each character, not pretty.
It is possible but not going to be easy. It may be easier to use graphic LCD instead of character LCD.