I’ve never completely understood this, but I think the answer would probably be “no,” although I’m not sure. Usually when I leave the house I turn off wifi and just use mobile data (this is a habit from my pre-VPN days), although I guess I should probably just keep it on since using strange Wi-Fi with a VPN is ok (unless someone at Starbucks is using the evil twin router trick . . . ?). I was generally under the impression that mobile data is harder to interfere with than Wi-Fi, but I could well be wrong and my notions out of date. So, if need be, please set me straight. 🙂
Note that VPN is just trusting a different network.
If you trust your VPN provider not to misuse your unencrypted traffic / inject exploits, but not your mobile phone provider (or any other network provider you might roam onto), then a VPN provider could help.
If you trust your VPN provider less than the mobile phone provider, the situation is reversed - you would be better not to use a VPN.
If you trust them equally, there is probably no point using a VPN (except for the roaming situation, which could be forced in certain circumstances).