On what basis?
A witness is not allowed to give an opinion on another witness, but Zimmerman is not a witness, he’s the defendant.
The basis is generally that fact witnesses are precluded from assuming the role of the jury. It is up to the jury to decide, for itself, whether or not it finds Zimmerman's account to be credible.
The "witness vs. defendant" distinction doesn't save that encroachment on the jury's turf. Probably makes it worse.
Would you feel the same way if Serino said he thought Zimmerman was a pathological liar?