This is very silly. I don’t think private correspondences ought to be held to the same standards as a writer’s published fiction; even bringing them up is nonsensical.
Beyond that, I’d encourage you to look up the definition of “puerile”, which is as much about being juvenile or silly as it is about sexual or scatological - say what you will about those letters or Joyce (or Nora’s!) particular fetishes, there’s nothing that suggests that they weren’t in earnest.
As a sibling comment at least alludes to, it’s much fairer to point out that Ulysses has plenty of its own sexual or scatological humour, and that someone might easily describe it as puerile. And fair enough; as to why it doesn’t personally strike me that way compared to Pynchon, all I’ll do is rest on the de gustibus defense.