I thought Prometheus and Covenant were both decent enough. It's not like they sunk to the levels of Alien 3 & 4.
I'd say I think you're confusing being pretty with quality. It's okay, a lot of people do it. Prometheus is lovely to look at. On every other level - script, dialogue, concept, plot, acting, creativity, vision, throughline, meaning, relevance, humanity, emotional impact, it is undoubtedly a far, far, far worse movie than Alien3, and I don't like Alien3.
Resurrection? That's more of a toss up. But at least I didn't feel I'd actively become more stupid because of the absolute mind-numbing idiocy of the plot and characters of Resurrection, whereas with Prometheus, I felt like I'd been bludgeoned by a conspiracy theorist half-wit to try and bring me down to his level. It was kind of like being beaten up by a beautiful but extremely stupid man.
Covenant is about equally bad to Resurrection in terms of terrible, hackneyed writing (not Whedon's best work), but has
even dumber ideas and is a lot less
fun, and given how dumb the ideas in Resurrection are, that's saying something. So Resurrection edges out Covenant. Also both Alien3 and Resurrection were attempts to do something different, whereas Prometheus and Covenant were both ultimately attempts to just try and recapture Alien, but with more cod philosophy and attempts at grand guignol which didn't actually work as well as they did in Alien (in part because so many of them were accidentally funny rather than scary or horrific).