He played there for the Netherlands and looked okay. Nothing more. He isn't going to be an attacking threat, but then Walker isn't on the right. He rarely does anything other than pass back/sideways and threaten to burst forwards before checking back.
Ultimately it's a problem position we can probably manage without reinforcements. Cancelo, Mendy and Ake can all play there and we can win games. We've seen Laporte play there before and we've managed. But if we want to become a CL winning team you do want to have the balance across the pitch without glaring weaknesses. The best teams will target our weak spots. Lyon did it last season and our manager was so concerned by them doing so he sacrificed the team's strengths to try and solve it.
Early days though, Mendy shouldn't really be judged until he's played 10 games this season. He's coming back from long spells out with injury and it will take time to get up to the pace of things again and develop. He's coming into the prime age for a full back, let's hope we get the best out of him.