Answer

Greetings.
First, the sages of the Oral Torah in the non-mystical realm also added things based on their own judgment and reasoning. There is no principled problem with that.
On the matter itself, it is not clear to me whether Kabbalah has a basis from Sinai or whether it is entirely a later development (that is what I am inclined to think). Even if it does have a root in Sinai, the later developments were not transmitted on a broad enough front, and therefore there is no good oversight of the results (as the Hazon Ish said about manuscripts uncovered from the genizot: since they did not undergo critique on a broad front, they do not have binding validity).
In my opinion, it contains interesting ideas and insights from people with good spiritual intuitions, and therefore one can certainly draw inspiration from it. But in my humble opinion, it has no binding authority. One who accepts it may accept it, and one who does not—does not. In other words, even someone who does not accept it at all would still be considered among those who observe Jewish law.