The next version of freeCodeCamp (version 7) is currently in development. I am hopeful that the new version will be more mobile friendly. This is somewhat limited by the resources available. For example, the code editor used by freeCodeCamp is the monaco editor This is is probably the best online editor available, but it does not do well with some virtual keyboards.
You can use the freeCodeCamp curriculum offline by cloning the GitHub repository and running your own version locally. Obviously, completing challenges on a local instance will not update your online account.