Institutional Shareholder Services (ISS) said it has "significant concerns" over the size of the award, up from $14.8m the year before.
Mr Cook, whose net worth is reportedly more than £1bn, received the pay in shares, salary, and for other costs.
The BBC has contacted Apple for comment.
In a letter to shareholders, the ISS said there are "significant concerns" over the "design and magnitude" of the package. "Half of the award lacks performance criteria," ISS said.