I'd be curious of why you think they are "dumb" to be fair.
Judging by how they look, the themes are pretty clear:
Prehistoric paradox pokemon are designed to be primordial, monstrous versions of the pokemon
Future paradox pokemon are designed to look robotic and artificial, potentially with a forced integration of metallic parts in their body.
You could say it's kinda stereotypical, which well, fair, they succeed in same way of the Ultrabeasts succeed in their design: they look unnatural. Appropriate for "paradoxes".
Ofc I'll be looking forward to what the actual fuck is going on with them in first place, since the design only makes sense if their lore also does. Are they from a future where life doesn't exist as we know it and everything has become made of machine parts? Are them a time paradox caused by some sort of infection?
You know that episode of SpongeBob where Squidward gets a time machine? That's what I feel like with the designs. They're both stereotypical yes, but past designs have more to offer imo: they looked like rugged monsters/cavemen while "everything is chrome in the future".