Sewn fabrics or fleeces of any type are not suitable to be left in cages with hamsters. They are OK for supervised use but I would never risk them in the cage. You may think your hamster does not chew then one night it discovers it can chew bedding and ends up with loads in the tummy or thread wrapped round the teeth or feet and choking or losing a limb
Even cotton wool pods, even though they are natural have very strong dense fibres which the hamster cannot digest quickly enough to prevent a gut blockage.
Take some loo roll and put it in water - see what happens? It dissolves very quickly into soft loose fibres. That would happen if a hamster ate it. Harmless.
Take a cotton wool ball and do the same. What happens?
Same applies to socks and fleeces and cotton material.