People use "upside down" and "mirrored" almost interchangeably, but they describe two different transformations, and mixing them up is the most common reason a flipped-text attempt looks wrong.
Upside down text takes each letter and rotates it a half-turn, the way a printed page looks if you flip it over. An "e" rotated 180° looks like an "ǝ". Because the whole line is also rotated, the character order reverses too — the last letter you typed appears first. This is what this site's generator produces, and it's the version that reads correctly if you were to physically turn your screen upside down.
Mirrored text instead reflects each letter across a vertical axis, like a reflection in a mirror — a "b" mirrored looks like a "d". The character order stays the same, left to right, only each letter's shape is reversed. This version reads correctly if you hold it up to an actual mirror, not if you rotate it.
A line built for one effect looks broken when someone expects the other — a mirrored line rotated 180° doesn't reconstruct the original text, it produces a third, unreadable jumble. If you're going for the classic "flip your phone upside down and it still makes sense" effect, that's the upside-down transformation, not mirroring.