The firm's proprietary survey leads Maxim to expect iPhone revenue to come up 14% short of consensus estimates in the fiscal second quarter through March and 6% below expectations for the fiscal year.
Those softer iPhone sales will pressure the company's profit margins even as its services and wearables divisions grow, Maxim said.
Apple shares on Thursday were trading just above $263. Maxim placed a $190 price target on the shares.
The firm estimates that fiscal 2020 services revenue would rise 18% from fiscal 2019 to $54.4 billion. That estimate is in line with Wall Street's consensus view of $54.3 billion.
Meanwhile, wearables and home and accessories revenue is expected to jump 31% to $31.2 billion.
Those totals represent, however, less than a third of the nearly $268 billion of revenue the company is expected to generate in fiscal 2020. The majority of Apple's revenue is still tied to iPhone sales.
Maxim analysts do say the adoption of 5G-capable devices will be a boon for the Cupertino, Calif., company.
"We see the 5G iPhone, expected to launch in September 2020, driving fiscal 2021 (through September) iPhone revenue up 4% year over year (vs. down 5% year over year for fiscal 2020 and fiscal 2022), despite our survey data indicating otherwise," the firm said.
"Regardless of the strength of the 5G cycle, we believe assigning peak multiples -- AAPL's p/e is now at 20 times consensus fiscal 2020 estimates, a five-year high -- to expected peak iPhone revenue is unjustified," Maxim's note said.