The interesting thing about mobile is that there are so many games being released that people don't even realize are new that there are always chances for new creators to get in. The problem is that many of them don't have very large audiences, but there is "some" demand. I would say, check out the multiplayer games, or new popular games and then search them up on YouTube to see how many people are making videos on them and how those videos did and from how long ago.
What you really want is popular and low competition, but usually this isn't easy to find so a lot of the time it's more like mildly popular with low competition and then once you have enough viewers you can tackle the games with more competition.
I'll mention the game I'm playing, Castle Crush, I think it still has room for some more YouTubers but the audience isn't huge and you have to know the right topics to make videos on I think. There's also a lot of content already made so better to focus on new cards/updates and challenge videos. The plus to Castle Crush is that it has an overlapping audience with Clash Royale. You're probably better off playing a newer game though.
Sometimes the best strategy for mobile though is like I said to just work through new games until you find one with demand and then jump on it as the expert to gain a quick audience. There isn't the same type of hype as with PC games so it can be easier to find games that get overlooked.