"Mind not high things" †
This means do not follow after fame or be ambitious for greatness and great deeds to feed pride.
Next clause refers either to associating with the common folk and poor OR being content with humble tasks. Either way, it is opposed to vainglory: the desire merely to impress people, often based on empty achievements, often aimed at those with status.
Does this mean we are not to aim for greatness at all.
Let us aim for greatness in goodness and in the quality of our service, our ministry, whether the job be big or small, glorious or unknown; and care less about the look of it to others. †