Maximum voltage divider resistances for sensing LiPo battery voltage over 10bit ADC: is MOhm range working?
By itself the impedance is too high. The AVR needs the impedance to be 10k or lower, otherwise the sampling capacitor does not have enough time to charge to the measured voltage and conversion result will be inaccurate.
If you put a capacitor at the ADC input pin, that is large enough (e.g. 100nF), it can provide the necessary AC impedance to the ADC input. However it will not affect the DC impedance so the conversion must not happen too often to let the capacitor voltage charge enough via the high impedance resistor divider.