Of course jailbreaking iOS puts it into some insecure state. Quite literally. Jailbreaking circumvents code signing for all code that runs on the device which means that every bit of code that makes its way onto the phone will happily run now. Also using the repositories means that you will install undocumented binary code from unknown people. Since you don't have the sources there is no way to check what this code does and since whoever wrote that code faces no risk when his code is discovered to be malware there's very little you can do after the fact.
This is less secure than a device that is not jailbroken.
I mean, do what you want to do by all means, but at least try to know what you're doing so you can correctly balance the risks and advantages you get by what you're doing.