MoPub also has a Marketplace feature which is a RTB exchange for mobile that competes against the ad networks you integrate.
Are you able to see any requests and impressions for your account from the AdMob integration?
You should be using your AdMob's 'Ad Unit ID' and not the 'Legacy Publisher ID.' I know it may be confusing as within MoPub's dashboard it mentions to have the 'Publisher ID' but really it should be the 'Ad Unit ID' from AdMob that should be entered.
Please consult this guide for more info: http://help.mopub.com/customer/portal/articles/1376821-how-to-set-up-updated-admob-revenue-reporting