An excellent post!Speaking as an airline crewmember for over 25 years, yes it is common. It is widely known as "Recovery Day". And then there is also "Prep Day" the day before the next trip.
Partly it is physical fatigue, and partly it is a psychological need to not be moving and not be on somebody else's timetable.
Re-read Deejo's comment above. While the details revolve around your spouse's job, the issue is whether she will attempt to modify things and whether it will be enough to keep you happy in the marriage. This will take some months to shake out.
She can (and should for her long term health) modify her sleep schedule on trips. If she is crossing many time zones she should attempt to remain on her home zone as much as possible even if it means being awake and active in the middle of the night at her destination. Careful use of light doses of melatonin and caffeine can help her regulate her wake/sleep cycles. But many trips have atrocious schedules, like red-eyes which completely mess up circadian rhythms. She should investigate working different types of trips even if it means having fewer total days off, less desirable trips, different and less desirable days off, or lower pay. She should control her food as much as possible by bringing healthy food and avoiding junky unhealthy purchased food. She should be sure she is staying very well hydrated.
I find having something specific planned when I get home helps prevent wasting a lot of time. Part of the psychological thing is, in a way, there are 2 very distinct lives going on. One is at home, the other is away. When we go on a trip we are away for several days, not just at the office for 8 hours. Your office is in your home area. Everything is the same. The radio station in your car, the local news, the weather, where you sleep, who you hang out with. For flight crew everything is different from home. So there is a period of adjusting back to being at home. On a trip everything is work. There's no home things going on. All the home things can only be done on the few days off. Couple the re-entry process with needing to not be on a schedule (to the minute) when things have to be done and it is easy to just vegetate for a day at home.
So try to get her to have something to do an hour or two after getting home, or first thing the next morning. Even if it is just a calm walk in the park or visiting a family member. Not a specific set time (like a movie), but a general plan. After dinner we'll walk the dog.
I would approach it with her as an undesirable part of her job, not as something she is failing at with you. Enlist her cooperation. If she understands that you'll be making breakfast in the morning (doing something yourself to help her) but then the plan is to go for a bike ride to get moving, she can see it as a team effort.
Having something in mind to do when she gets home will help. She'll be thinking about it during the trip and especially on the last leg home. So she'll be visualizing being home even while on the trip which speeds up re-entry.
I want to add just one thing. I have a friend who is a professional skier, and is in Europe for 4-5 months in the winter away from home. It means the world to him to come to his girlfriend who supports him and always has his favorite cookies waiting for him.... My thinking here is that your wife lives for the support you can give her in a job that requires a lot of travel.