It’s just a superficial liner that helps protect the actual window. I admit that it looks embarrassing and unprofessional, but there’s no safety risk.
And Dai Whittingham, Chief Executive of the UK Flight Safety Commitee, told the paper:
This is definitely not a safety concern, it is simply a piece of loose cabin trim.
The internal window is there to protect the main glass from scratches and it cuts down some of the internal noise as well as providing an element of thermal insulation.
The external window is fixed to the hull and sealed with a gasket so that the interior of the aircraft can be pressurised, so this loose trim will have had no effect at all apart from not looking particularly neat and tidy.