Apple shares fell more than 2 percent Monday, falling for a second-straight day due to mounting concerns about unsustainably high stock prices. The iPhone maker's stock has lost nearly 6.2 percent of its value in just two days.

The decline Monday came after Mizuho downgraded the stock to neutral from buy and lowered its price target to $150 from $160.



"We believe enthusiasm around the upcoming product cycle is fully captured at current levels, with limited upside from here on out," Mizuho managing director, Americas research, Abhey Lamba, said in a Sunday note.



Apple's stock fell 3.9 percent Friday as major tech stocks suddenly plunged. Shares closed 2.39 percent lower Monday at $145.42, after hitting a session low of $142.51.

The stock closed at $154.99 on Thursday before the big selling began.

Apple shares three-month performance

Lamba said investors already expect strong iPhone 8 sales in the coming product cycle, likely limiting gains in Apple shares. And the same consumers driving those solid sales this time around will likely not spend on a new iPhone soon, reducing potential growth in fiscal 2018, the report said.

Customers may also want to wait for the iPhone to adopt new OLED display technology at a lower cost, the note said.



"In our view, Apple's pricing is likely to become the main hurdle to meaningful expansion of its installed base," Lamba said.



He also expects that growth in China "is likely to remain weak" in the near term, while the Indian market remains constrained due to limited affordability. The consensus estimate of 30 percent growth in services revenue per user over the next two years also "seems high," Lamba said.

"We remain of the view that Apple maintains a very strong franchise and could continue to gain incremental share of the smartphone market; however, the pace of share gains will likely meaningfully moderate as the market becomes more saturated," the report said.

Lamba does not assume any repatriation of Apple's more than $170 billion overseas cash holdings, which he expects will be taxed.

Apple, the largest market-cap stock in the S&P 500, ranks third worldwide by the value of short interest in its stock at $9.1 billion, just behind Alibaba at $16.7 billion and Tesla at $10.5 billion, according to financial analytics firm S3 Partners.

In May, Aswath Damodaran, a professor of corporate finance and valuation at New York University's Stern School of Business, said Apple's stock is fully valued around $140 a share, while $94 is a good place to buy.

Some on Wall Street also believe Apple's recent downward volatility is a negative signal for the market.

"I'm not sure what the right value is for Apple, but it is extremely concerning to me, that a stock of this importance can trade like that – on virtually nothing!" Brean Capital's Peter Tchir wrote in a note to clients Monday.

"After Friday's price action, I can't be anything but bearish near term. The 'healthy' rotation seems to hide the fact that for brief moments of time there was virtually no liquidity (at a price) for some of the biggest and most loved stocks," Tchir added.

— CNBC's Tae Kim contributed to this story.