Quote 1
She went over to the window seat for her cigarettes, lit one, and returned to her seat on the bed. "Mother?" she said, exhaling smoke. (1.44)
Notice that Muriel is indoors, and surrounded by smoke and nail polish. The atmosphere is very different from that in which Seymour is placed – outside, on the beach, in the sun and the clean air.
Quote 2
"I just got here, Mother. This is the first vacation I've had in years, and I'm not going to just pack everything and come home," said the girl. (1.60)
Muriel's reasons for staying are essentially selfish; she isn't considering Seymour or his well-being.
Quote 3
"I couldn't travel now anyway. I'm so sunburned I can hardly move." (1.60)
Some critics think that the sun/sun block/paleness of various characters is a metaphor for different degrees of innocence. See "Symbols, Imagery, Allegory" for more.