Also literally every pokemon eats berries in the games.
Also many creatures who are otherwise primarily carnivorous still eat fruit when available.
Also Trapinch may not have the visual acuity to tell one thing falling into its dinner trap from another.
Also one of the most common other pokemon where Trapinch live is Cacnea so even when its traps DO result in murder it probably still ends up with a fair amount of plant matter in its diet.

Also this is adorable.