Getting matched to Clans which are much stronger than your own Clan is very awkward. We all don’t want lose the win streak we have been working on for months, don’t want to see your TH9 villages get ripped by TH10 with Heroes level 40 and maxed army compositions. My mini TH8 Clan got matched with a TH9 Clan last week and obviously we had no chance to win the War. If you don’t want to be matched with a much stronger Clan, you should follow the Mustaka’s advice.
Basically when you start searching for a Clan War, you join a queue with other Clans which are also looking to be matched. As a result, the longer time you need to wait, the broader that queue is. Normally all War mismatch happens because it takes a very long time (more than 1 hour) to get a match.
You should reset the War Search every 10 minutes (no more than 15 minutes) and your Clan will not get mismatch anymore.