Is cydia safe? To be completely honest it is not 100% secure. The main reason is because most of the jailbroken apps that are submitted to the Cydia marketplace are approved. Which could leave your phone open to malware, however I have never heard of a case like this and if it did happen im sure that they would catch on quick and would have their source or become banned from the cydia marketplace.
How to avid unsafe tweaks and themes from Cydia
There is one very simple way to avoid downloading any unsafe material from Cydia to your android device is to search out reviews on Google. Most of the tweaks
and themes will have something written about them and you can also find more in-depth review on YouTube
My Tip: If you are really concerned the best way to cover yourself is to only stick to the trusted repo’s/sources that are already on your device once jailbroken such as (big boss and ModMyi, etc..). alternatively if you dont want to do this you can do some research into weather the specific source is trusted or not. you would only run into trouble if you download a repo/source that no one has ever heard of before or does not come with the original jailbreak.
So in conclusion to the question: Is cydia safe? It is just as safe as owning a PC or even MAC as there are no known cases so fare however jailbreaking your device does leave it open to a lack of iphone security.