"I'll ask them what they would think had he worn a tshirt with 'Kathleen Wynne forever' on it?"

Now THAT would be a risk to impartiality.  He's an Ontario provincial justice, and might very well be asked to rule on cases involving Wynne herself, Wynne's party, Wynne's MPPs, or Wynne's government or LPO voters.  There's a very real possibility of a conflict of interest and a biased ruling, in that case.

But exactly how many cases is he going to rule on this year that have any spectre of involving Trump, the Republicans, or the US Federal Government or Trump voters, given that he's an Ontario Provincial Justice? Conflicts of interest typically require a possible biased ruling.  If that biased ruling isn't even truly a possibility due to context, there's no possible compromise to the justice's impartiality.  It's unprofessional, of course, but this wasn't going to have any real world impact on his cases. Nor could it.

Give him a slap on the wrist and let him get back to work.