Digressing in Footnotes
While I’m a big believer in “ask your adviser” about many questions, it’s useful to remember (and to remind yourself occasionally) that this is your thesis. You are the author. With that status comes the freedom (in a technical sense, at least) to write literally whatever you want in the thesis. That’s what it means to be the author of something.

So yes, you don’t want to annoy your adviser and other readers of your thesis too much, but remember: it is your thesis. If it makes you happy to discuss Einstein’s metaphysical beliefs (or lack thereof), there’s no need to overthink the issue. You worked very hard for several years to come up with interesting things to say, the least that people can do to respect that monumental effort is to humor you by putting up with a few not-very-relevant side remarks.