In this case yes because the website itself has a dark-mode toggle in the top right corner, and in its dark mode, the images are not visible.

The site automatically displays in dark mode if the browser says it’s using dark mode.

So this isn’t something the user is doing to manipulate the style and layout: their browser is saying “hey, fyi, this user’s local system biases to dark mode” and the site is choosing to respond by styling in a way that breaks diagram visibility.

This site has a theme picker to toggle between light and dark modes.

The blog has a toggle for darkmode and some of their images are black text with a transparent background. When darkmode is toggled, the text is effectively invisible, so in this case it seems to be an oversight of the blog.

This isn't the case of a browser plugin modifying the styles. The blog framework or whatever detects what your browser/system preference is and respects it. So if you've got your browser/os set to "dark mode" the page renders in "dark mode". Except the author used transparent images with dark lines, so they are invisible.

I think it's fair enough to complain about.