How to fix iCloud Storage is Full on iPhone
You can free up storage in iCloud by deleting content that you're not using:
Reduce the size of your iCloud Backup.
Delete photos in iCloud Photos.
Delete folders or files in iCloud Drive.
Delete texts and attachments in Messages.
Delete messages and manage Mail.
Delete voice memos.
Often the Apple Mail app is to blame for a full iCloud storage. Large attachments can eat up lots of space. When deleting old emails and removing corresponding attachments, you recover storage space