I think the default Breeze theme could use some changes to look cleaner. In these before and after pictures (Plasma 5.10) I changed the following:
- Changed the color scheme to Breeze Light. This makes the active titlebar the same color as the window.
- Removed the circle on the close button. It doesn't blend in, in my opinion.
- Disabled the gradient on the title bar. This looks problematic with Breeze Light.

One of side benefits of Breeze Light is that the breeze theme for GTK applications doesn't need to change at all.
One more thing, the theme probably appears better in 5.12 because of the increase of shadow on the sides of the windows.

Before:


After:


EDIT: changed the before picture

I fully agree with removing the circle from the close button and removing the gradient by default
Every time I install a KDE distro I turn these things off first.
I read many posts about people complaining about these and the default plasma/breeze look.
Yes, plasma is very versatile in themes but the default look is what draws in people.

This does look a lot cleaner indeed. I like how the title bar blends in with the UI better. It actually reminds me a lot more of macOS than Gnome (which is a good thing I guess).