Literal translation
he thinks himself greater than Master Moucho
Meaning
he thinks he's more important than he is
Explanation
The expression compares someone's vanity to that of a figure called Mèste Moucho, a character from Provençal popular folklore evoked as a type of hollow importance or contested authority, on the same pattern as other invented proper names, such as Master Arnaud or Saint Cabedèu, that the language popularized to build vivid comparisons without targeting any real person. Since the name itself is not attested in the project's dictionaries, it is not translated: it works as a culturally transmitted reference whose precise origin has faded while its comic function remains intact, that of embodying an exaggerated pretension against which the target of the remark is mockingly measured. The construction se crèi mai que (to think oneself more than) is very common for teasing someone's pride or self-importance, and appears with various other complements in many Provençal sayings. The register is openly mocking, reserved for familiar conversation among acquaintances, never for formal use.