In South Park episodes, Kenny's mom usually wears a t-shirt which says "I'm with stupid", and an arrow pointing towards a person standing next to her. How would you translate "I'm with stupid" into Russian?
Russian speakers usually don't include themselves into such an expression. The second meaning of it if written on a T-shirt ('my companion is stupid' or 'I accompany this psycho' - этот придурок - со мной) is lost here:
этот придурок прав (= а ведь прав этот придурок)
тупой, а соображает (= он хоть и тупой, а иногда соображает)
A bit different situation, but it is more typical in Russian to indulgently agree with a stupid person in anything they do or say:
... И не оспоривай глупца. (А.С. Пушкин, "Памятник")
что с него (дурака) возьмёшь?!
дуракам закон не писан
if i get the meaning of the phrase, for example
Я согласен с этим дурачком
the problem with this rendering is that it's not unisex
to circumvent this limitation a unisex pejorative may be used, but the adjective is still gender specific
Я согласен с этим чучелом
a more wordy but gender neurtal version would be
Мы единомышленники с этим чучелом
I would translate:
Рядом с тупым (I'm near a stupid person) or
Тупица рядом (A stupid person is next to me), or even more succinct:
Тупица (a dumb person), which is a slightly derogatory (but not overly offensive) word for a stupid person in Russian common language. For example, one could call their sibling
Тупица to tease them and this won't be too inappropriate.