In The Effect of Gamma Rays on Man-in-the-Moon Marigolds, the "radiation effects" (the gamma rays) on the plants are analogous to the hostile and stressful effects Beatrice has on her children. Ruth seems to have received high amounts of this metaphoric radiation and Tillie has received moderate amounts. Describing the differences between exposures, Tillie says,
The seeds which received little radiation have grown to plants which are normal in appearance. The seeds which received moderate radiation gave rise to mutations such as double blooms, giant stems, and variegated leaves. The seeds closest to the gamma source were killed or yielded dwarf plants. (99)
Tillie hasn't been killed or dwarfed, but she certainly isn't normal either. With the latter, you can look at this two ways. One is that she does get made fun of because her intelligence makes her stick out and because other students likely hear their parents and teachers speak disparagingly of Beatrice. Ironically, Beatrice is constantly worrying about how her children make her look bad, when in fact, it is Beatice's behavior that has made her children's lives more difficult.
Tillie is also not quite normal in a potentially positive way, and this is analogous to the benefits that may come from mutations. Like the double bloom and variegated leaves, Tillie herself can grow and/or learn from the angry effects of Beatrice's radiation. In this way, mutation (changing in response to an abnormal or inhospitable environment) can result in the effected organism (the marigolds or Tillie) becoming something new and wonderful. Mutating literally becomes triumphing in the face of adversity.