This is a tricky question. Sometimes when people are in conflict it can appear as if the world is against them. Only those who loudly endorse their view of the dispute can be trusted. Everyone else is working against them!
Although there are occasions where a mediator may display bias, there are also situations where bias may be perceived. I would advise you to speak up. If something has happened during the mediation session that causes you concern speak to the mediator. Give the mediator the opportunity to explain or change their behaviour.
Although mediators are often described as neutral, in fact many of them are omni-partial i.e. on everyone’s side. This can be confusing for parties. In reality the mediator is not biased for or against either party, their goal is to help the parties come to a resolution. At the end of the day, the mediator is only an advocate for the process.