To me, all-lowercase text comes across one of two ways:

(a) I felt speed was far more important than readability (reasonable for rapid-fire short messages or constrained typing ability such as a flip phone, also a common way to imply "fuck you, my time is more important than yours" in longer forms such as email), or

(b) i'm 14 and e e cummings is so deep (blogs)

(c) i may be jeffery epstien