Hmm. Generally I don't feel very easy about it.
Some random thoughts. Note that this is not a thought-out analysis, but just some things that immediately come to mind.
Who decides who is 'expert' or not? If the expertise is narrow enough there may not even be anyone else around who has sufficient relevant expertise to act as a confirmatory "Yup, he's an expert" or a "Nope, he's a fool".
There's a little too much possibility for self-promotion here too. An observation I've made is that anyone who chooses a user name that includes words like "export", "wizard", "master", academic titles, and other such like terms is usually (but not infallibly always) a walking talking example of the Dunning-Kruger effect. i.e. Knows enough to be dangerous and over-confident in their own abilities, but actually has at best half a clue. So self nomination is right out in my book.
But if people are nominated by others then there's a risk that they may not want that nomination. That would need handling carefully to avoid all round embarrassment.
Also it could push a lot of work in the direction of nominated 'experts'. I don't think I particularly stand out from the crowd as one of the 'go to' guys but I already get the odd PM out of the blue from other forum users along the lines of "I saw you'd written several times about X and I've got a related problem Y, can you help". Generally if I can I do, but the requests are few and far between. Now if, god forbid, I got the tag of "Go to man for turbo-encabulator expertise" I can imagine that the PMs might flow thick and fast. It needs some thought about how to handle this to prevent people ending up with a part-time unpaid consultancy gig. It's one thing to chirp up in a topic if you're in the mood and have useful expertise to impart, and if you're not in the mood you can just pass. It's not quite so easy if you've got a personal appeal for help sitting in your PMs.