While modern AI models handle busy backgrounds reasonably well, a plainer background still tends to produce slightly more reliable, cleaner transformation results.
Clearer subject-background separation
A plain or simple background makes it easier and more reliable for the model to identify exactly where the subject ends and the background begins — a genuinely busy or visually complex background can occasionally introduce small artifacts at those edges.
Since the background is being replaced anyway
In an app like a club-kit transformation, the entire background gets replaced with a stadium scene regardless — a cleaner source background gives the model a simpler starting point for that full replacement.
Practical, easy options
A plain wall, an open sky, or simply a slightly blurred background (natural depth of field from standing a bit further from a background) all work well without needing any special setup.
Does it matter if there are other people in the background of the source photo?
It can slightly complicate the model's job identifying and isolating the intended subject — a photo with just you clearly in frame, without other people prominently visible, is the more reliable choice.