One of the nice things about Qt5 (as far as I understand) is that it provides better support for HiDPI (retina) displays, but at the moment it doesn't look like Matplotlib is taking advantage of this:

PyQt4:

PyQt5:

MacOS X:

Note that MacOS X does take advantage of the HiDPI display, though the icons are still low-res

Interestingly, the low resolution pixels in the PyQt5 example look crisper which suggests that it might be rendered at HiDPI but the content of the canvas is low resolution.

So just to clarify, I think there are two issues here:

• The icons themselves are not HiDPI-friendly (as seen in the MacOS X screenshot) [update: have opened an issue for that: Use higher-resolution icons on HiDPI-friendly backends #4978]
• Matplotlib is not taking advantage of the HiDPI with the PyQt5 canvas