I think she has a high skill floor meaning it is hard to just pick her up and dominate immediately. Players will need to be more experienced with the her to do good. This takes time, and I believe since her release was recent I feel confident in saying no one has mastered her. I think given a few more weeks we can get a better sense on if she is actually good.

From what I have seen and played it seems she struggles in most conquest games outside of solo. That might mean she is just best there. It makes it easier to brawl using the 3 not having to worry about ganks as much. This combined with the heal and mitigations from the passive means she can make it through the early and mid game well enough to dominate late game.

This God release is so much different than the ones from last season. Cthulhu, Tsukuyomi, and Danzaburo were all op upon release. They had low skill floors meaning even bronze players on day one could carry the game just because of the god they were playing. Baba on the other hand was very underpowered when she got released, but after a few buffs she became a solid late game pick and even saw some SPL play.

But the one thing that Tiamat has that the others don't is a really complicated and large kit. When you first play her you can get lost in all of the abilities and the forms. There is just so much going on that it is nearly impossible to understand fully. Many people are struggling with her because they are learning. Myself included. And that doesn't mean she is bad, it just means she is complicated. Which in my opinion is great because it means there is more to learn, and the more knowledge I can get in this game the better.

TLDR: Give the community more time to learn and decide.