Low fill-rate in Admob is normal for Appodeal setup because Admob counts the requests for each ad unit.
When you synced your Admob account, the Appodeal plugin creates multiple ad units on your behalf and set different price floors for each ad unit. Ad units with higher price floors tend to produce lower fill rate. However, each ad format has the ad units without price floors whose performance should be always close to 100%. Check Appodeal dashboard to see the actual fill rate because we count the requests to a specific network.