Dermot Mulroney believes Sydney Sweeney is so much more than a pretty face -- despite the fact some believe she's just getting by in showbiz on good looks and a rockin' bod.

We caught up with the actor in L.A. Tuesday ... where he addressed Hollywood producer Carol Baum's controversial criticism of his "Anyone But You" costar. As you may have heard ... Carol blasted Sydney's acting ability and looks, slamming 'ABY' as being unwatchable.

While Dermot laughed off Carol's hate for his and Sydney's rom-com -- in which he played the actress' father -- he did issue a reprimand to Carol ... making it clear he doesn't approve of the way she dissed his costar's looks.

As Dermot puts it ... Sydney couldn't be a better collaborator, praising the bombshell actress for her warmth and likability. He adds ... "She's more deserving than anyone I know. She's worked her tail off since she was a kid and she is exceptional in every way."

This is a similar sentiment to the one shared by Sydney's former acting coach, Scott Sedita ... who praised the actress' inner beauty and talent to us not too long ago.

Of course, the "Euphoria" star's team also hit back at Carol's criticism ... labeling the producer's comments as "sad" and "shameful." Carol herself has walked it all back.

Anyway, Dermot has Sydney's back here ... and he cosigns the notion that she's got chops and then some -- wisely noting there's a lot of hot people in this town, but not all of them are crushing in H'wood the way Sydney is. Solid point ... and we're sure SS appreciates it.