Carries a negative nuance: describes an idea, solution, or approach that is overly simple, shallow, or lacking depth. Often used critically.
His proposal is too simplistic and not realistic.
You can't solve this problem with a simplistic way of thinking.
Dated usage for something inexpensive or cheaply made. Modern Japanese more often uses 安い or 安っぽい.
The goods in this shop are cheap, but the quality is only so-so.