If this is for the high score slide, first copy the default slide provided in the GMC addon code into your own slide folder, then continue editing from there --
What I think I am seeing in the screenshot is the default high score input, which has a scale factor set. The pixelation is due to font rendering, so to fix this you need to reset the scale factor and increase the font size.
The player name display can be done directly via the Theme Overrides settings, after resetting Right Panel > Control > Transform > Scale x and y to 1.0.
The letter choice text must have Scale reset to 1.0 as well, but theme overrides dont work here, so you must define a new theme. On the theme, you select the Label type, go to the Font Size submenu, and change font_size to something larger.