From the Super Mario Wiki, the Mario encyclopedia
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In the game, when Mario, Luigi, Baby Mario, and Baby Luigi reach the top of Thwomp Volcano, the volcano erupts. Thanks to E. Gadd's help, however, they manage put the fire out. When the Mario brothers reach the top of the volcano again, they find out that the Thwomp couple waited every year for the volcano to erupt, but since Mario Luigi and the babies had stopped the eruption, Mr. Thwomp becomes depressed, despite his wife's attempts to cheer him up. Mrs. Thwomp then blames the Mario brothers for calming the volcano, and engages them in battle.
After defeating Mrs. Thwomp, Mr. Thwomp got angry at the quartet for hurting her. However, after explaining their story, Mr. Thwomp offered to take Mario and company into Thwomp Caverns. But when Mrs. Thwomp found out he was taking the heroes into the volcano, she asks him if she can come too, and then gets angry when her husband says she is "too heavy." In the Thwomp Caverns, Mr. Thwomp comments that Mrs. Thwomp is scary when she is angry, and when Mario and co. explain about Princess Peach, he takes to calling her "pretty lady" and whenever the bros talk to Mr. Thwomp again, he will always offer to take them into the volcano.