Why does the hidden service of facebook (http://facebookcorewwwi.onion) ask for a mobile phone verification? Isn't the goal of TOR to be able to visit pages which are censored in some countries (e.g. from China) anonymously? This does not make sense to me.
Facebook allows neither anonymous nor pseudonymous accounts. That means you can't sign into Facebook as a guest, nor can you use a name that isn't your real name. It then follows that when you visit Facebook, either on the clearnet or via Tor, you have to sign in with your own credentials.
If you're signing in with your own credentials, then Facebook has to check it's actually you, and not someone breaking into your account. (I'm sure you'll agree that this check is a Good Thing.)
If you're using the Facebook hidden service (i.e. .onion site) via the Tor Browser, then Facebook is unable to validate you in the way it normally would (e.g. using cookies, your location implied by your IP address, etc.), and so requires another method, in this case via your phone.