The whole point of Overwatch‘s Lucioball mode is that everyone plays as Lucio. The name of the mode is already “Lucioball,” the only character you can normally select is Lucio; it seems pretty transparent that you are only supposed to play as Lucio. Some enterprising players have found a way to exploit a bug to select characters other than Lucio despite this fact, because gamers are alike all over.
A recent forum post has confirmed that you really shouldn’t do that.
Director Jeff Kaplan has confirmed that the team is aware of the bug and is trying to fix it, but players should not use the exploit for two reasons. First, it negatively impacts the experience of other players; two, because the customer service team is looking to hand out penalties to players who exploit the bug. So don’t do it. Enjoy Lucioball as it’s meant to be played (hint: with Lucio) and don’t try to exploit bugs. It’s just more fun in the long run.