Dylan Efron isn’t claiming mama’s boy status ... but he is stepping up big time to back Ashley Tisdale in the middle of that toxic mom group chaos!

Zac Efron’s little bro popped up on "Watch What Happens Live" when a caller put him on the spot about the controversy, and he made one thing crystal clear -- he can’t imagine his brother’s former 'HSM costar ever doing anything malicious, saying Ashley was nothing but sweet to him growing up.

He even tried to smooth things over, suggesting the whole scandal might just be a giant misunderstanding ... adding he genuinely can’t see where the drama would come from on her end.

Andy Cohen, of course, wasn’t about to let it go. He jokingly poked the bear, saying maybe Ashley had too much time on her hands after getting dumped from the mom group ... and that’s where the chaos came from.

The chat stayed light and playful ... but the drama was very real earlier this month when Ashley dropped an essay that online sleuths think was about the group she shared with Hilary Duff, Mandy Moore, and Meghan Trainor.

Ashley’s camp later insisted she wasn’t talking about any of those celebs ... but Hilary’s husband clearly wasn’t buying it, firing back with a post slamming a "major, self-obsessed, tone-deaf person."