Important. Anyone can design a header image or change widget configurations, but a really good designer and design team can infuse your brand into the platform.
I have to mention that a designer is nothing without the front-end developers who help build the design. There are many levels of customizations available on every version of the software and a solid team can mean the difference between your community coming to life or stagnating.
Ask a your Jive Rep who they would recommend. Ask for examples of the best designed communities. Remember that some companies are just interested in platforming you and not helping with custom strategy, design, development, and activation. Your Rep can help.
Another option would be attending Jive World. All the biggest vendors are there showing the best, most transformational uses cases.