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.
Mr Cook, 61, who has often spoken publicly about his concerns over equality and human rights issues, said in 2015 that he would give away his entire fortune before he dies.
According to ISS, Mr Cook's pay was 1,447 times more than the wage of an average Apple employee.
His package included $630,600 in personal security costs and $712,500 for personal use of a private jet. ISS said the cost of such perks "significantly exceeded" comparable companies last year.