Mages are already quite powerful due to CC (and some high tier spells), but if you want to change the cap you can adjust the value in the 'Data.txt' file (key "Skill Primary Attribute Max After Boost","150").
If you have downloaded the Steam version of the Editor you can find the file in:
'Steam\SteamApps\common\Divinity Original Sin Enhanced Edition\Data\Editor\Templates\Stats\Shared\Generated\Data'.
If not, you can download it from the link Cadmus88 provided in this thread (just scroll down to my post 'Questions and answers' #582573):
Guide: Editing companion starting propertiesWhen you changed the value in 'Data.txt' you must store the file in:
'Steam\SteamApps\common\Divinity Original Sin Enhanced Edition\Data\Public\Shared\Stats\Generated\Data' (GOG version should have the same folder structure). If the folders don't exist, create them.