It's a nice tool that provides some basic raster "painting". Functionally, it's quite similar to Microsoft Paint for older Windows versions (think Windows 95 through Windows XP). I use Microsoft Paint to create pixel art as a hobby, and unfortunately, Paintbrush does fall a bit short for it to be a replacement.
For one, the floating window application design has never been a good design paradigm, and it doesn't work for a tool like this. If we take a look at more advanced tools like Photoshop and Pixelmator, we can see that floating windows have been abandoned a long time ago. Even GIMP has gone for a single-window design in more recent builds.
Two, to build off of point 1, the overall UI and other design language could be brought into the current MacOS design language. I would highly suggest using Preview as an example.
Three, the tool is missing a few features found in modern Microsoft Paint. Microsoft Paint, going back to Windows 7, has added quite a few additional shapes and antialiasing (even though this isn't something I use, I would appreciate it being there). Additionally, a feature found even in older versions of paint, the hidden color replacement tool with the eraser tool, is missing.
Four, the color picker tool does not work as expected. In Paint, the you select the color you want to replace with the picker (either the primary or secondary color), select the color picker tool, and then pick the color on the canvas you want. PaintBrush, counter-intuitively, has you "left click" for the primary color and "right click" for the secondary color. It should follow the behavior of Microsoft Paint.
These are nitpicks in the grand scheme of things, though. It may be unusable to me for pixel art because of the design choices and tool functionality, but for most people whom I'm assuming are installing for nostalgia reasons or for their kids, it's great. It just needs my complaints mentioned above for it to be a tool for me.