Q. Does poor tajweed invalidate salah?

Answer: It all depends on the type of mistake that is/has being made. If the mistake is such that it changes the meaning of the word and the change is of such a degree that it causes Kufr, or the error changes the meaning so as to accuse Allah (Subhanahu wa ta’ala) of saying something He has never ever stated whilst committing Kufr, then it such an instance, the salah would have to be repeated.

For example, a person states that good people would go in to Hell, instead of saying: good people would go to heaven, then this would naturally negate salah. Also if a person changes certain letters that would cause the meaning to change drastically, then is such an instance a person would have to repeat the salah as well.

If the mistake is small enough where the meaning does not change, then the salah would be valid.

In both cases, merely approach the Imaam in private and address your concerns to him with valid proof.

Allah certainly knows best.