Hi I am wondering if a wool cot underlay is safe for a 6 week old baby and then continue to use as she grows.

Red Nose recommends that a cot mattress be firm, clean, well-fitting & flat.
Some woolen underlays take away from the firmness of the mattress &/or are loose with the potential to create a “trough” on the mattress.
A soft mattress or sleeping surface can increase the risk of sudden unexpected infant death if baby rolls over onto the tummy.
Remove plastic packaging from the mattress and always make sure that the waterproof mattress protector is strong and a tight fit.
Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft.
https://rednose.org.au/article/what-is-a-safe-mattress