I suppose we have to call this a feature.
In the HTML the elements are placed in order:
online dot - avatar - username
In desktop style the avatar is placed in the margin via CSS.
In mobile style all elements get back to their original ordering, which puts the online dot at the start of the line.
No idea if it's intentional or if nobody ever noticed at that time but it looks like it was made to look like that.
Now I do agree it's not very pretty... Good news is, the ordering has been changed ("fixed") for beta8: flarum/core57f828b