They have not been formally condemned as a group to be heretics, so no. The people making up the NO are at worst material heretics.
Now, if you know individuals who are formal heretics, by all means avoid praying with them.
THIS^^^^ (with one exception). Generally speaking, those in the NO believe that they are Catholics and have not therefore professed separation from the Catholic Church. That is what makes the NO heresy even more insidious. It's not like, "I'm going to become Greek Orthodox." People still profess adherence to the Catholic Church.
No, "at worst" there are many formal heretics in the Novus Ordo. I would venture to say that 95% of them are. By their own polls, the vast majority of them reject one or another Catholic dogma, that they know to be dogma, because the mentality of the NO is their feeling entitled to be "cafeteria" Catholics, which basically is formal heresy, since they do not accept the teaching of the Church as their formal rule of faith (THAT is what "formal heresy" means ... and has nothing to do with "sincerity").