I wrote the following to support@mullvad.net:
Hi,
Any updates on this?
On https://stacker.news, this issue comes up time and time again that you don't support LN payments yet. For example, see here: #146849 or here #71743. There is a website where one can buy vouchers using LN: https://vpn.sovereign.engineering/ Are you aware of this website? Maybe you can add it to https://mullvad.net/de/help/partnerships-and-resellers/ so more people are aware of it. Would also be cool if you mention it when someone wants to pay with Bitcoin since he can pay with LN using this website.
I really love your VPN btw! Keep up the good work, the only thing that's missing is really just LN payments (as I mentioned before).
Sincerely, ekzyis
I replied to their answer in September that they don't support LN but they might in the future.