the english version of the material Im reading says "that's a mean god", and a friend told me that means "there is a bad god ,isn't it?" , but I don't understand why. I know ひどい神 (bad,cruel, God) I guess the もんだ its a contraction of ものだ , but I still don't get it , what is もいた? , can someone explain all of this in detail please.
It's (酷い + 神) + も + 居た【いた】 + (もん + だ).
もんだ at the end is a contraction of ものだ, and this type of ものだ carries the sense of exclamation or surprise. So the sentence means something like "Wow, a bad God exists!" or "I'm surprised at the fact that such a cruel god exists!"
EDIT: For this "past tense" いた, please see Usage of plain i-adjectives or た form (悪かったｖ悪い、良かったｖいい etc） and https://japanese.stackexchange.com/a/40888/5010
EDIT 2: For this も, please see: Odd use of も has me stumped All in all, this is a combination of three (mild) exclamatory expressions!