Are woollen cot mattress toppers safe? im looking at purchasing a topper with wool filling and organic cotton covering.

Red Nose recommends that a cot mattress is firm, well-fitting & flat & that a mattress protector is strong & a tight fit.
A soft mattress or sleeping surface can increase the risk of sudden unexpected infant death if baby rolls over onto the tummy.
https://rednose.org.au/article/what-is-a-safe-mattress
A woollen underlay would not be suitable if it reduced the firmness of the mattress or was not a tight fit.
This is particularly important when baby is rolling.