I'm not going to try to explain mission readiness, this is low, but the target is closer to 70%, so while it isn't great, it's not like it's half of what's expected.
Jets are complicated, they spend a lot of time in maintenance and inspection for each out of flight, its because they're optimized for performance while passenger jets are optimized for endurance, safety and cost.
F35s are terrible jets for peacetime, as are most fighter jets. For combat they're probably a lot better, they're basically f16s with worse maneuverability, but better range, sensors, weapons and of course stealth, 1 f35 is worth at least 3 f16s.
But even this doesn't matter, the next generation will be 80% drones, drones have phenomenal mission readiness ratings, often 90% or higher.