the phrase "fat mood" does not have a specific or widely recognized meaning in English. However, it's important to note that language evolves over time, and new slang terms or phrases may have emerged since then.
In general, the term "mood" in internet and social media culture is often used to express a relatable feeling or emotion. People may use phrases like "big mood," "in the mood," or "mood" in a sentence to convey that they strongly identify with a particular sentiment or situation.
Without more context, it's challenging to determine the exact meaning of "fat mood." It's possible that it could be a new slang term, an inside joke, or a phrase specific to a certain community or group. To get a more accurate understanding, I recommend checking more recent sources or asking the person who used the phrase for clarification.