When your ac source is above 1.5v, the mcu output side will be high.
When your ac is below 1.5v, your mcu output side will be low.
Now we can see what you are suggesting is basically one of these "sub 10 component trigger circuits".
And therein lies the problem. Why is no one else triggering at 1.5V? Everyone goes for about 10V+ and a pulse width between 100us and 1ms and takes the mid point. Why?
David Hess put it very simply above. The mains are noisy. If you put a super low voltage triggering and super fast optocoupler in there. It may well trigger multiple times..all over the place. Then you need to LP filter that....blah blah. I haven't drawn up your proposed circuit, but just from following the words it sounds like this would trigger on positive cycle only? Ie I only get one edge. So no pulse width averaging? Maybe not needed but given the spread but given the noise issue, might have allowed some safety reject logic...again, more complications.
The AC optocoupler turns on and off an LED based on current in the LED, not voltage. It works by the voltage going though a resistor illuminating the LED strong enough for a 'transfer' to the detector side of the optocoupler. This only shows a small window opening while the LED doesn't have enough current to shine bright enough to drive the photo-transistor. This also doesn't tell you the current AC phase which might be useful.
The opto-isolator part and connection example I recommended goes high when the AC phase goes effectively above 1v into the positive, and goes low when the AC phase goes below 1v down into the negative. The optional 100pf cap will do a little cleaning, but I cannot imagine the design messing up or getting noisy as this is so close to the 0 transition point, really bad noise from loads like light dimmers and switching supplies occur above or below this voltage window where it is too late on the digital output transition has already taken place.
Make no mistake, instead of the data isolator, you can use a RE1C002UN or SSM3K35AMFV (mosfet 1vgs switch on) or equivilant small signal mosfet & diode protection clams on it's gate input & using it's Drain to drive a normal optocoupler's led & you would get similar results.
You would still need a RE1C002UN or SSM3K35AMFV (mosfet 1vgs switch on) , 6N137 or equiv, 1N4007 diode, a 5v zener diode, a 22k resistor for powering the zener, a 470k resistor to drive the gate, a 1k ohm resistor to drive the optocoupler LED at ~4ma, 1 47uf 6.3-16v cap, a BAT54S double diode for gate clamp protection, and an optional 100-1000pf cap for the AC line filter tied to gate & GND. That's 9 components.
All components are bottom end cheap ones.
The data isolator part count is 6 components, however, the data isolators cost much more alone than the 9 components listed above.