Actually “thick” in english can be used to mean stupid, so it’s sort of a pun on the normal phrase “Thick as Thieves,” which is used to mean “closely allied with”. So it sounds like they at least translated a possible meaning, without preserving the pun, which might not have been possible. I’m a little rusty on german idioms related to thieves, I’ll admit.
Well, back in school days our they´ve told us that “to be thick as thieves” means “to be tight friends with someone”. If I assume that they were right, german idioms like “Wie Pech und Schwefel” or “Dicke Freunde” would have been more suitable translations.
Yeah I think the joke with "“Thick as Thieves” is that MPQ makes out the Maggia goos to be pretty dumb. So in english you could also read it as “Stupid as Thieves” in the same way as “Thick as a brick” ie dumb as a post.