IMHO the appropriate focus there is that whatever argument you can make in the form that "criminals should get a standard of life of at least X" would also imply that non-criminals also should get at least the same standard of life. Prisoners do deserve all kinds of things, but to be fair, everyone else does as well. And if you assert that a certain thing should be provided for criminals but you already have a non-trivial underclass who can't afford these things and have no plans to provide those things to the non-criminals, then it is a reasonable source of frustration for a honest poor individual who is being denied that opportunity - why is society abandoning them and is literally treating them worse than criminals, if they have done nothing wrong?

And it acts as an anti-deterrent if someone sees that they would improve their material conditions by doing harmful things and going to prison. There's nothing wrong with ensuring good prison conditions, but it has to go hand in hand with ensuring equally good non-prison conditions for poor people.