In SmartMenus 6 there was the following property in “c_config.js”:
0, // ItemsSeparatorSpacing
but I suppose tweaking this will not fix the issue and most probably some of your other page styles are conflicting with the SmartMenus styles and overriding them. So if possible, please post an URL to some kind of live demo where I could investigate the issue. Otherwise, it’s impossible to guess what exactly might be causing it.