The Tudors definitely smeared him. But at the same time in the 20th century there has been a little too much in the other way. I thoroughly approve of the reevaluation of him. But sometimes people are a little too quick to credit him. Things like deposing his own nephew are excused as realpolitik today but that was absolutely not looked upon well in most of England at the time.

We only have a few things to go by if we want to completely divorce ourselves from post-Bosworth work:

Most significantly, Richard III faced a rebellion by former Yorkists in 1483 led by the Duke of Buckingham and it gained a substantial following. The fact that they were Yorkists is crucial, of course, because he is a Yorkist. This was not just aftershocks of the War of the Roses, it was his own party. The original goal of this was to place Edward V back on the throne. Only when the rumor that he was dead got around did they decide on Henry Tudor. Now facing a rebellion doesn't mean much (Henry Tudor had to put down rebellions too) but it certainly doesn't suggest overwhelming popularity either. And the base of the rebellion was fairly large, which is a significant point. There was definitely still unrest in the kingdom which had been mostly at rest under Edward IV's second reign.

His principle advisers, Catesby, Ratcliffe and Lovell, were widely detested. There were such widespread rumors about him (that he poisoned his wife, that he had people murdered) that he had to demand that anyone who uttered them be arrested. This is not a good sign. All kings have rumors but it suggests these were either widespread or particularly troubling that he actually had to address them and demand the gossipers be arrested.

On the other hand, he was widely beloved in the North, where he spent most of his career. And he was remembered for being a smart and fair lawgiver well after his death. He was without a doubt an able commander, personally brave, and very intelligent (and say what you will about that wicked Tudor propagandist William Shakespeare I think all of those qualities shine in his Richard III and I often wonder if for all of the OTT villainy and propaganda in the play there wasn't more than a kernel of truth in this charismatic, intelligent, and funny character). I think he was probably a very good king in many ways. In many ways he was truly the last Plantagenet. He was capable of tremendous self-serving wickedness but also had a serious mind to justice and fair law giving (which was true of Henry II, John, Edward I).

Whether he murdered his nephews has become kind of a huge distraction in the discussion of Richard III, in my opinion. Of course, it's significant and goes toward his character. But the fact is he had no more legal right to the throne than Henry VII. The evidence of the princes not being rightful heirs because Edward IV married someone else was flimsy, at best. And didn't arise until well after Edward had died. He was a usurper. At least Henry VII took his crown in battle and married Edward's daughter. And, while they absolutely smeared him thoroughly, the Tudors didn't have to do much to make that unpalatable. Young Edward V retained loyalty and sympathy. Then again, Henry IV was a usurper. Edward IV, his own brother, was a usurper. It is worse to do that to a child but it's hardly an unforgivable sin in the English monarchy.

I'm completely laying off of the murder of the princes issue. It's become too much of a hotbed. And I think it's absolutely impossible to know the truth or even what people thought was the truth. It's been too smeared with the later Tudor accounts. As I said above, I think it's something of a distraction anyway. Taking their throne was the crucial thing.

Sources: The Winter King by Thomas Penn (on Henry VII but goes into much of this); War of the Roses Dan Jones; War of the Roses Alison Weir (I know, I know but it's very accessible).