Does Humility affect the graveyard?
No, it only affects creatures, which is to say creature cards and tokens on the battlefield.

109.2. If a spell or ability uses a description of an object that includes a card type or subtype, but doesn’t include the word “card,” “spell,” “source,” or “scheme,” it means a permanent of that card type or subtype on the battlefield.
110.1. A permanent is a card or token on the battlefield. [...]