Despite being a Practiced skill card that can potentially give upwards of +4 to +6 to an investigator's Will or Lore, Occult Theory is largely made irrelevant by its restriction on only having icons when it is still in your hand or committed to a skill test. Since Occult Theory has no icons outside of those select circumstances, Occult Theory becomes a dead card when attached to cards like Astronomical Atlas and is never a valid target for events like Practice Makes Perfect or even Written in the Stars (since those cards have no icons while they're still in your deck). The card is still potentially playable as a Hail Mary for Investigators that need flexible skill dedications, but there are no actual combos or tech involving Occult Theory when Mystics already have access to numerous great Level 0 skill cards.
At the same time, Occult Theory compares poorly against Level 0 Mystic cards and doesn't provide good enough stats to meaningfully compare with other upgrades that cost experience. Investigators with access to Mystic Level 0-1 who need a large dedication to pass a Will-based skill check can just cancel treacheries like Ward of Protection and Deny Existence, while Mystics who need to gather clues can just use their will to investigate using cards like Rite of Seeking or Read the Signs rather than try to boost their Lore to compete with Seekers. And even in the event that an investigator still desperately needed a flexible skill dedication, Mystics already have amazing skill cards like Promise of Power that give +4 to every stat that they can filter out of their deck with Practice Makes Perfect or Astronomical Atlas. Occult Theory is the answer to a non-existent problem for Mystics, and even in the event that an Investigator wanted to try the card out, they should already have access to strictly better option within the Mystic card pool.