Older Y/A book. Coming of age. Girl is transformed over the course of a summer

[Kayla]

Hi! I’m hoping someone remembers this book I read as a young teen in the late 1980s. Here’s what I recall:
• The cover showed a heavyset teenage girl sitting at a diner counter drinking a milkshake, wearing a striped shirt.
• She feels unpopular and self‑conscious, especially about her weight and acne.
• One summer, her older cousin visits and teaches her how to wash her face, care for her skin, eat healthier, and gives her a haircut.
• She changes and gains confidence
• It was text-only (no illustrations) and I borrowed it from the library around 1987–1989, so I suspect it was published in the late 70s - early 80s.
• It had the vibe of a Scholastic Wildfire, Bantam Sweet Dreams, or Starfire teen romance of that era.

Does this ring a bell for anyone? The cover art and plot have been stuck in my head for years!

Thank you!!

[Becca]

Outta Sight by Mary Blakeslee?