3.3V Oscillator AC couple to 1.5 Vp-p input
You might want to make your question a bit more clear by stating what the MICRF220 does, it's a "300 MHz to 450 MHz ASK/OOK receiver".

This is the typical application diagram:


Your question is about the (clock) reference input, it has a Crystal connected in the diagram above. The question is, what's inside the chip connected to RO1 and RO2. Fortunately the datasheet answers that on page 11:



This is a fairly typical example of a crystal oscillator circuit inside many ICs. If you want to apply an external clock you could try to input a clock signal (with a voltage that varies between the voltage at the IC's supply pins) at pin RO2. I find the suggestion of applying the clock to RO1 as is done in the datasheet a bit strange as that would work against the inverter that is present between RO1 and RO2.

You could just try this with a current limiting resistor (so no AC coupling) in series with the clock so that you cannot damage anything. I would apply the clock to RO2 via a 10 kohm resistor. Then measure with an oscilloscope at RO1, if you get a large clock signal at RO1 then that's good. If not try applying the signal at RO1 and try if you get a clock at RO2.

So yes, you can use DC coupling but try that first with a series resistor of 10 kohm.
The AC coupling is probably only needed when the signal is directly (without series resistor) applied to RO1 as RO1 is internally biased to a DC voltage needed for the crystal oscillator circuit to work.