No, it almost definitely isn't. The cost-benefit analysis of doing that completely fails at almost every imaginable level.
Not only would it fail to increase American nuclear strike/retaliation capabilities by any meaningful margin, but if discovered it would set off a massive nuclear arms race in space which would be massively destabilizing and benefits no-one which is exactly why all the major players signed that "no nukes in space" treaty in the first place.
> It's a great first strike weapon without a detectable missile launch. If terrorists are building a nuke you can drop it on them and make it look like they blew themselves up. Last ditch asteroid defense.
No to all points. Nukes don't just unintentionally go off and hypothetically if the did people on the ground wouldn't normally expect the very noticeable trail of a reentry vehicle screaming in from space immediately beforehand. That kind of thing is very noticeable, go look at ICBM test footage. If you want to nuke someone without warning you use stealthy cruise missiles because those don't tend to get noticed until they detonate. Any nuke carried by the X-37 also lacks the delta V to ever get to any dangerous asteroid you might want to try to intercept.