I see that too. I suspect it's down to an art director with bad taste more than anything. The watery pastel stuff is extremely hard to explain. It's not all bad, but almost none of it is remotely memorable or engaging. The covers are higher-energy for sure and generally better but a lot of them are still vague and unmemorable and just don't have even the "oomph" of covers I genuinely dislike. Ok the MM has a good cover. But the PHB? What is even happening? I guess there's maybe a fire giant, but if I wasn't a D&D player I would have no idea what that even was. And an elf(?!) is doing some sort of trampoline jump in front of it whilst glowing in a totally non-threatening way? Or the giant has invisible strings and the elf is a puppet? Man what. I literally just noticed that dude in the bottom right with the sword too. This is a mess of a cover. It kind of looks like everyone is scared of everyone else and falling over or something.
Even 3E's UTTERLY VILE covers which gave me a sort of phobic "don't touch that!" reaction were at least memorable and identifiable. 4E's were less memorable but, I dunno, at least there were cool adventurers on them. Not some sort of vague "Accident involving a Fire Giant".
What I find hard to explain is why certain covers grab me while others are so... meh. But, I think it comes down to a memorable image that captures both the spirit of the book and the mood of the art. Whereas the PHB's cover seems neither incredibly empowering, very tense, or really all that interesting, the art of the DMG shows the evil Acererak standing atop a mountain of skulls and bodies, all of them beginning to come to life, somewhere in an ancient grey wasteland where ruins from a forgotten era dot the landscape. It perfectly captures the role of the DM as the master of plots, plotting, villains, and the sheer immensity of the world.
Likewise, the MM shows a group of hopeless heroes running from a variety of terrifying monsters in a dark and stormy ruin, their hopes dashed. It captures the power of enemies and the role of monsters in the game.
The PHB's cover, however, does not capture heroism, and doesn't clearly show a cohesive and exciting adventuring party. Instead, we see a group of panicked adventurers jumping around and nearly being killed by a massive Giant who takes up the majority of the composition. Instead, perhaps the party should be together, facing an enemy with strength, rather than terror. Even the cover of Pathfinder's Core Rulebook better represents the role of an adventuring party.
The splatbooks have had hit-or-miss covers, in my opinion.
VGtM has a memorable cover, as the grey color scheme sets it apart, and we can clearly see a lone hero (well, it is Volo, so maybe, not a hero) negotiating with a Frost Giant, thick tome in hand. The cover itself helps inform how the information in VGtM helps DMs make the monsters seem like something that can be spoken to and experienced in terms of culture, rather than just homogeneously evil bags of HP.
Other splatbooks accomplished less.
