This is called Visual Hierarchy. Cubase does provide a generally very good Information Hierarchy at its core, and in complex designs like this, optimizing for the needs of most users is a wonderful and difficult challenge. Cubase has done really great work here and that work is still in play.
However, as you correctly point out, Cubase 13 suffers from a mismatch between the Visual and Information hierarchies it provides, which makes navigating the UI much harder and requires users to put in a lot of work to actually find and use the valuable Information Hierarchy beneath it. A Visual Hierarchy is meant to expose and magnify the Information Hierarchy, not further obscure it. As mentioned, it is already very difficult to structure a solid Information Hierarchy in the first place, and Information Hierarchy is the most critical element of navigating complex apps… under no circumstance do you want a Visual Hierarchy that destroys the tremendous time and effort put into your primary usability offering–Information Hierarchy!
Cubase 13 has a bit of an identity crisis in this regard. On the one hand, brand new features like the improved multi-part editor with visibility tab and the new channel tab show a practical understanding of Information Hierarchy needs and their value to the product. These features expose specific data in a context aware and meaningfully placed way, one that understands there is a bigger picture and that some things should be in the background while others should be front and center. In fact, the Visual Hierarchy for these features clearly separates degrees of prominence and places some items in greater importance over others. (For example, both the visibility tab and channel tab are appropriately made secondary to other views.) So, in this case, Cubase 13 got it right.
On the other hand, the Cubase 13 “look” as the OP called it, or the “theme” as many others call it, is a different story. An overly-flattened visual design downplays and even removes Visual Hierarchy entirely. When Visual Hierarchy is undermined, much of the hard work that was done to build an intelligent Information Hierarchy is also undermined. Suddenly your hopes of communicating a logical and organized interface are jeopardized by careless visual design decisions. A casual and irresponsible Visual Hierarchy can instantly take the heart out of a wonderfully designed Information Hierarchy.
Many people, and sadly even younger, less experienced designers, don’t comprehend the criticality of good visual design. Many in this thread have talked about the visual design as only a matter of opinion–“a preferential coat of paint” so to speak. There is room for preference here for sure, in fact, quite a lot of room. But reducing visual design, or shall we call it “look design” or “theme design,” to a matter of preference is highly ignorant and neglects decades of industry research and objectively provable science. You can go in almost infinite directions visually (i.e. lots of room for preference), but good designers will always find and constrain specific areas based on research results (i.e. preference has nothing to do with it).
Cubase is full of excellent designs and poor designs. I don’t have firsthand knowledge of internal Cubase team structures but, the product results point to silo’d ownership and/or designers of varying skill levels leading projects independently. It would be nice if more senior folks were responsible for the primary Cubase UI, as it’s the flagship interface, and less senior folks could get their bearings on integrated plugins, configuration screens, and other less traffic-heavy and highly critical UIs. Alternatively, there might be business struggles or other financial or technical constraints that “forced” Steinberg to provide a release that wasn’t ready (i.e. a visual design update done without the research requested by senior designers). Unfortunately, someone made a bad call internally and some poor design language found its way into the flagship screens. The good news is, the areas where Visual Hierarchy was made worse in Cubase 13 are actually quite easy to correct. Here’s hoping Steinberg improves things in the next few minor updates.