No, it's not.
A designer making that mistake isn't a signal s/he is a bad designer. I've worked with UI designers who range from good to excellent when it comes to design.
Thus, my overarching point which was that UX ≠ UI
and that UX shouldn't be co-opted by UI teams.
Similarly, you seem to be using a particularly narrow definition of "designer" here, as if the only people designing web UIs come from print backgrounds and the only thing covered by "design" is aesthetics.
If you have a team where you have someone from that mostly unrelated background doing your UI design, sure, they're obviously not going to make great decisions. But even then, there's still no reason to assume your programmers will do any better, unless those programmers happen to also have genuine UI design skills (which they certainly could, but it's mostly orthogonal to their role as programmers).
> you seem to be using a particularly narrow definition of "designer" here
My definition of UI designer is somebody who designs UI (which encompasses those of print background which a lot will probably have been at some stage in the/their past)
> but it's mostly orthogonal to their role as programmers
No, it's not, unless your programmers have been relegated to code monkeys. In years gone past (pre web dominance, when native desktop applications ruled), developers were designers/user experience etc.. which lead to the checkboxes everywhere UI.
Good UI needs someone with good design aesthetic. This is where UI designers shine.
In contrast to other design fields(most notably Industrial Design, where designers have pretty much always owned (& thought about) design & interaction as a whole), that is not something which UI designers have done. It's only in recent years that I've seen UI teams claim UX in the form of renaming themselves as "UI/UX team". My point is that UX has not historically been something they've put much thought into ,and as a result, they are not qualified to just own UX.
But UI has always been about more than mere aesthetics. A UI is literally how the user interacts with the system, the interface between them. That subject has always encompassed usability and accessibility issues, information architecture, planning sequences of interactions, and so on. What you've described is exactly the illusory distinction I was talking about.
I'm not sure there's much to be gained from pursuing this line of discussion any further. Obviously you can redefine terms to suit your argument and thus make your argument true in that context, but then it doesn't really address the original point of debate, which was whether developers should be overriding designers and domain experts in matters of design. If you relegate your designers and domain experts to the equivalents of those code monkeys you mentioned while elevating your developers to developers who are also designers and domain experts, obviously the latter are going to make better calls on most UI issues as well, but then you have to ask why you had separate designers and domain experts involved at all. I don't think that's a normal distribution of skills and responsibilities within a product team, though.
in matters of UX...
Do designers who design webpages/apps generally call themselves GUI designers?
To answer you question, no. Such people usually design interactions. Behavior. What happens when the user clicks or scrolls or drags. So they're designing UI and they're called UI designers. UX is a step removed, defining the overarching design language for the app and how it makes users feel.
In my experience there are no people who work only on the visual layout of a page and not on behavior (what you incorrectly called UI). Mockup artists maybe, but even they are usually thinking about behavior.