There has always been great debate about what exactly design is and isn’t and I’m not going to get into that on here but I think it’s safe to say that there are a few things that good design definitely isn’t, one of them being decoration.
I’ve been amazed at how often those outside the discipline of design assume that what designers do is decoration—likely because so much bad design simply is decoration. Good design isn’t. Good design is problem solving. — Jeffrey Veen
A good designer isn’t someone who just ‘makes things pretty’ at the end of the process, they should be involved from day one and have an influence on the direction of the project from the start. In today’s modern responsive web decisions are often made on the fly and designing itself is done a lot in-browser (as opposed to static mockups in Photoshop or something similar). A good designer who understands the relationship between design and coding and how modern browsers and devices render today’s code is an essential part of any well designed product on the web.
There can also be a temptation to hire a designer to simply apply the latest design trend or fad over your wireframes but this is (in my mind) the wrong approach. The design process has many facets and I think the end ‘look’ of your website should be as a result of consultation, research, planning and an understanding of the unique requirements and audience of your project. Blindly following trends is not only a way to make the end product date very quickly but also shows a lack of confidence in the product in the first place.