Does the background in my selfies matter?

Not much. The background is replaced by the style you get, so a messy room does not show up in the result. What matters is that your face is evenly lit and not blocked.

Backlit selfies are the classic failure: with a window behind you your face is in shadow, and there is very little detail for the model to work from.

Low light and heavy in-app filters cost more accuracy than a modest phone camera does.