Appearances can be quite superficial. Just because someone seems nice, or looks beautiful, doesn't mean that they actually are. Johnson's poem focuses on the way that appearances can mislead us. If we look good on the outside, it doesn't mean we're good on the inside.
Lines 83-86: The imagery of the "painted face" and the "the frame of gold" evokes a painting or a portrait. It also suggests the way in which people make themselves up to look good. These lines suggest how looks can disguise the corruption that's within.
Lines 319-320: Even mothers are into appearances: they want their child to have "the fortune of a face"—to look beautiful. The speaker suggests that such a superficial focus on appearances isn't a good thing. We should care that our children are good, not that they look good.
Lines 323-324: The speaker's words in this section suggests that good looks don't necessarily go with wisdom. Our good looks will lead us towards pleasure rather than toward wisdom.
Lines 341-342: Appearances—and especially our beauty—can lead us toward temptation. And temptation, in turn, can lead us into disgrace. These lines continue the poem's emphasis on the dangers of appearances.