Win rate isn't really a good indicator for whether or not they need to make any changes to a champion. Think of it this way, since Tahm is the newest champion in the game, that means that people are still trying him out and learning to play him right? Well since a lot of people are trying him out and learning his kit, that also means there are a lot of people who are losing games with him because they don't know how to play him properly yet. People usually have a harder time winning with champions they're still learning vs. champions they've already mastered. The combination of people who are still trying to figure him out and people who lost a couple games with him then decided that they didn't like him, are usually enough to lower most champions win rates when they first come out. The same thing usually happens when Riot makes any significant buffs/nerfs to any champion they haven't touched in a while. People start playing them again and deciding whether or not they want to pick that particular champ up again.
I find that win rate is only really a decent indicator on champions that have not been directly or indirectly affected by any changes to the game in a couple months or so. So just give Tahm a bit more time, his win rate should somewhat stabilize after the wave of people who want to learn his kit and all of these pretty heavy changes that Riot has been making to the game lately die down a bit.
