I've been able to hitchhike easily on the east side of the Sierra Nevada in many location but I've never tried to in the Mammoth area specifically. My guess is it won't be an issue. But a fallback could be Mammoth Taxi. ?I used their airport service in 2017 and they do trailhead shuttles. Devil's Postpile is quoted at $75.?
https://www.mammoth-taxi.com/rates.php
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