As defined by the Cambridge online dictionary, being the low man on the totem pole is "someone who has the least important position in an organization". In the example you show, the organization would probably be the social network surrounding the wimpy kid. The dictionary goes on to show an example of how someone can start from being a low man on the totem pole and "work their way up to become a manager". I.e. moving your way up the totem pole is increasing your standing in a given group.
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