If the person producing the mockups does have an understanding of the client's requirements, the overall solution, and the overall and underlying system, and is also acting as the analyst getting the requirements from the client and coming up with a solution that fits within the client's budget, then all well and good.
But if the person producing the mockups is just drawing anything they want, focusing on the visual/graphic design of the public facing website and disregarding all the other important stuff, then that is not so good.
I've seen a few recurring problems with graphic designers acting as the overall solution designer, designing screen mockups in packages like photoshop:
1. Graphic designers creating designs without any consideration for the features and constraints of the underlying cms or ecommerce to be used, which can make it costly to retrofit the actual system. They might also do so with no discussion whatsoever about the customer's overall ecommerce requirements including backend ecommerce requirements. When a website is designed with no particular
underlying system in mind, it can end up being very costly, because the system then has to be either developed from scratch or developed in a semi-bespoke way, either way development time is expensive, or worse still it forces the use of an inappropriate underlying system because that is the only one that fits in with the visual design, even thought it's not a good solution design fit. This situation also happens with businesses just selecting web templates because they like the 'design', without considering overall website requirements.
2. They add things like Newsletter signup links to the graphic design without any consideration or discussion with the customer about what the
newsletter system requirements are or what the features should be.
3. Creating photoshop designs that are either impossible web designs, or designs which add to the budget, or designs which diminish the SEO of web sites, or designs which diminish what can be content manageable, or designs which do not convey functionality. For example: using fonts that are not web fonts, using text anti-aliasing that is not present in all browsers, using impractical font sizes, etc, generally designing for the print or image medium (which has no constraints) rather than designing for the
web medium (which does have constraints)