I think it's a perfectly normal strategy in a trial to try to introduce doubt in the minds of the jury about someone's testimony, it happens all the time. Experts are frequently wrong when it comes to evidence that's subjectively interpreted, for eg, there's been some scandals about bad science in interpreting how fires were started, that have been used to convict probably innocent people.
On the other hand, DNA evidence is convincing, not because of the qualifications of the witness presenting the evidence, but because of the scientific reliability of a DNA match, and the overwhelmingly convincing statistics about it's uniqueness in each individual.