Oy, what a question.
My response is based mostly in Jewish law, somewhat in personal experience, and slightly in common sense.
Kabbalah is generally defined as Jewish mysticism; the attempt to engage in an intense and direct Divine experience using certain techniques, practices and texts rooted in Jewish practice and literature. It has a long history, stretching back to the book of Ezekiel and the stories of the Talmud. The most famous story of the Talmud emphasizes the dangers of an ungrounded attempt to experience the Divine for those who are not rooted to a firm foundation.
“Four men entered the pardes —Ben Azzai, Ben Zoma, Acher [that is, Elisha], and Rabbi Akiva. Ben Azzai looked and died; Ben Zoma looked and went mad; Acher destroyed the plants (and became an apostate); Akiba entered in peace and departed in peace.” (Hagigah 14a)
The pardes is a Hebrew word meaning ‘garden’ but it is understood here to mean the direct presence of God. The story underscores that the ecstatic experience of the Divine is so intense that only the most highly trained can even attempt it, and even of these four Jewish scholars, only one emerges intact. Later Kabbalists, like Abraham Abulafia, Isaac Luria, and Yosef Karo were all trained masters of Jewish law and texts before attempting to apply Kabbalistic methods to their learning. As such, a commonly repeated injunction suggested that a man be 40 years old before he engaged in the study of Kabbalah, based on the quote in Pirkei Avot that “a man of 40 is ready for Binah – understanding…" (Avot 5:24).
On top of this, the central text of Kabbalah, the Zohar, is an allegorical and wildly digressive commentary on the five books of the Torah, and broadly references Jewish law, Talmud, commentary and Midrash. My own limited study of the Zohar for two years, with two seasoned experts in the field, still left me mostly befuddled.
The type of Kabbalah being practiced by Madonna, Britney Spears, and Naomi Campbell comes by way of an organization called the ‘Kabbalah Learning Centre’, based in Los Angeles. Back at the turn of the 20th century, a rabbi in Israel named Yehuda Ashlag claimed that the sources of Kabbalah were such universal principles that anyone could study them. Another rabbi, Philip Berg, took this idea and started an organization for Jews and non-Jews alike based on some abstracted principles of Kabbalah. Some of their practices are rooted in Jewish rituals, but taken in a different direction. Some of them are new inventions.
Some examples- Jews have a custom of rising on their toes during the words ‘Kadosh, kadosh, kadosh’ (holy, holy, holy) in prayer, to emulate the angels referred to in the prayer. At the Kabbalah Centre, during prayer times, practioners leap three times in the air. Jews in traditional synagogues study Torah for its meaning in order to get closer to God and what God wants of us. At the Kabbalah Centre, the belief is that ‘scanning’ the Hebrew letters - just glancing at their shapes, without understanding how to read - is enough to derive holiness. Text study, learning, observance of Mitzvot and helping others through Tzedakah and Gmilut Hasadim are generally de-emphasized at the Kabbalah Centre. In exchange, practioners focus on spirituality, meditation, and the inward experience of God.
So what is the draw to a celebrity? I would say it is the same thing that draws anyone- it is a peaceful, calming, meditative series of practices that emphasizes the self and your individual connection to the divine. From a more critical perspective, it asks for the individual to disengage from the messiness of life- poverty, homelessness, hunger and suffering of others, and focus only on the self.
So according to Judaism, are they ‘legitimately’ practicing Kabbalah? According to Rabbi Ashlag? Yes. According to all the Kabbalists and Talmudists who came before him, who envisioned Kabbalah as the pinnacle of years of study of the entire depth and breadth of the Jewish tradition, after total mastery of Torah, Tanakh, Talmud, and Midrash, and punctilious observance of all of the commandments and thrice daily prayer of the greatest meaning and depth? No.
But who are we to judge? It may not be the way we practice or recognize normative Judaism. But if a person finds meaning in it, it calms them, and makes them a better person, as my grandmother would say, “Zei gezundter heiht”, they should live and be well.