A screenshot of Roku channels with free content - why is the new Plex icon the tackiest one?

That’s why it’s tagged with a Roku.

If that Icon is really bothering you - install the Preview App.
It’s much less tacky:

… AND you get the benefit of early ‘features/fixes’ that, just may, at some point, provide a tacky Preview Poster or fix the tackiness of the Regular Joe App.