In a word, no.
(Even though you didn't tell us the name of your state...)
And, frankly, it's irrelevant. STBX's attorney is SUPPOSED to be working for the benefit of his/her client. They should rely on all the information the client gives them. Whether the attorney is a MIL, sister, 23rd cousin or Mickey Mouse doesn't change that.
However, it COULD work to OP's advantage. STBX's mother is likely to have a distorted view of the situation since she will likely believe everything that her daughter tells her. If a factual dispute arises, STBX's attorney/mother will likely go too far in stating their case - and then if OP has evidence contradicting it, it makes them look bad. So, the same thing applies here as in any divorce. If there are disputed issues, gather all the factual evidence you have and give it to your attorney.
Note, however, that while OP can't contest the DIVORCE on those grounds, he is certainly free to dispute any errors in the divorce filing.