I absolutely believe Oliver is impartial. The reason is because if the Republicans had elected a mild mannered, responsible and fair president, Oliver would have nothing to say about him. Maybe good things. If said president made a huge blunder, Oliver would rip on him for it.
If Clinton was made president and her bullshit carried on he would be calling her on it and most of his shows would be based on mocking her and calling her out. There's no doubt.
Just because Trump is a goldmine doesn't mean he's impartial. Hell, he'd be an idiot not to point out the ridiculous shit that Trump is doing and saying on a daily basis. The right doesn't do it because they're brand loyal, but it's sure as fuck true that Trump deserves every little bit.