I doubt there is an addon which can change the text of ALL addons.
But maybe one can kind of "overwrite" the standard defined fonts and sizes of the game somehow so the game and addon using this will use the overwritten/changed ones all the same.
I'll have a look if I can find something which does this already. I remember there once was some addon trying this.
Edit
Here are some adons which might help you as they are able to change the fonts of some of the game contents:
NamePlater:
https://www.esoui.com/downloads/info...ylesSizes.html
pChat fixed (some errors at least) by Baertram:
https://www.dropbox.com/s/2tazkm1n1i...rtram.zip?dl=0
Azurah:
https://www.esoui.com/downloads/info...eEnhanced.html