Of course they could afford it, but would it be good use of money? I kind of doubt a Mozilla-hosted VPN would be much more popular than the Mullvad-based one, and them partnering companies that do a good job instead of competing sounds like not a bad thing.
Not sure about exact numbers, but e.g. Mullvad appears to have less than 25 employees, some of which are going to be administrative overhead Mozilla already covers for the entire company, so lets say ~20 people on the product (including support etc)? Even a now downsized Mozilla has to be able to dedicate 20 people to an attempt to make money. And that is probably not as small as you could do it if you had to.
Don’t VPNs have a very large number of servers all over the world? That must cost a ton of money and make it hard for new players to enter the market without a significant investment. I’m not sure Mozilla can spare such an investment right now.