What Plain Text is
Plain text here means columns lined up with spaces, the way a command line tool prints a table.
Terminal output, log files, fixed-width exports from older systems.
What JPEG is
A lossy image format designed for photographs, where losing a little detail in a gradient is invisible.
Places that will not accept a PNG upload.
What changes when you convert Plain Text to JPEG
Two shapes are handled. If the same character positions are blank on every line, those gutters are treated as column boundaries, so values containing single spaces stay intact. Otherwise the lines are split on runs of whitespace.
The same render as PNG, saved as JPEG. It is the worse choice for a table and it is worth knowing why: JPEG compresses by discarding detail around sharp edges, and a table is nothing but sharp edges. Letters get a visible halo and hairline rules blur. It also has no transparency, so a background colour is always painted rather than left clear.
What carries over from Plain Text to JPEG
plain text records 2 things about a table that JPEG has no way to hold.
Nothing in JPEG records that the first row is a header. It is drawn at the top and looks like one, which is all that is left of the distinction.
JPEG is the end of the line for this data. Reading the table back out of it is a separate and much harder problem than writing it, so keep the plain text if you will need the values again.