Learn Spanish (Latin America): Ella no piensa que este estampado sea muy estético. - She doesn't think this print is very aesthetic.

Because of the negative trigger no piensa que. When verbs of thought or opinion (like pensar or creer) are used in the negative, they deny a fact or introduce doubt, which requires the subjunctive in the following clause. If the sentence were affirmative (Ella piensa que...), you would use the indicative es.

Why use a form of ser (sea) rather than estar (esté) to talk about how the print looks?

Even though we are talking about an appearance or personal opinion, being estético (aesthetic or visually pleasing) is considered an inherent characteristic of the design itself. Since it describes the fundamental nature of the print rather than a temporary physical state, we use ser.

Could I say no cree que instead of no piensa que?

Yes, they are essentially interchangeable in this context. Both no piensa que (doesn't think that) and no cree que (doesn't believe that) express a negative opinion, and both act as triggers requiring the subjunctive verb in the dependent clause.

What exactly does estampado refer to in Spanish?