Depends - what are you trying to do?
Mockups can be good for raising an investment (although not easy to do without an MVP and some data). They can also help the developers better understand your needs. But it all depends on the type of platform, who is doing the coding, your budget and numerous other factors. I built a MVP version of a marketplace without mockups because the design layout was fairly easy to understand - so no need. But again - it depends what you're building...
If you have the money, and someone you trust, it can't hurt - but first understand why you need it.

Good luck

I'm a mentor on Startups.com, an international lecturer on entrepreneurship and negotiations, a commercial startup lawyer and a leadership coach. I've successfully helped over 400 entrepreneurs, startups and businesses and I would be happy to help you.