Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
So, Linda has a tricky job. She has to make emotional pleas for abolition, but she also wants to make sharp, pointed critiques of the whole institution of slavery—including Northern complicities. In other words: how is she going to make her point without alienating her readers?
Well, one strategy is employing both methods at once or one after the other, to maximize her chances of succeeding with the reader. Linda often uses exclamations such as “O, reader,” when she's going after the emotional appeal: “O, what days and nights of fear and sorrow that man caused me! Reader […] I do it to kindle a flame of compassion in your hearts for my sisters who are still in bondage, suffering as I once suffered” (5.4).
But then she'll sharpen that up with a catchy, biting aphorism, like “Cruelty is contagious in uncivilized communities” (9.6), or "Hot weather brings out snakes and slaveholders" (34.10). She's also not afraid to lay on the sarcasm, as when she writes, of the rare slaveholder who is a good Christian, "Her religion was not a garb put on for Sunday, and laid aside till Sunday returned again” (9.13).
Basically, this seems to be a narrator who can't make up her mind whether she wants to lecture her readers or make them cry. It could be that this uneven tone helps explain why Incidents wasn't immediately popular.