Learn Spanish (Latin America): Este capricho ridículo es insoportable.

We use ser (es) because being ridiculous and unbearable are presented as core, defining characteristics of this specific whim. Even though it is an opinion, you use ser when describing the fundamental nature or traits of a person, thing, or idea.

I recognize the English word 'support' in insoportable. Is that what it means?

No, it actually means 'unbearable' or 'intolerable.' It comes from the verb soportar, which is a false friend. Soportar means 'to bear,' 'to tolerate,' or 'to put up with.' If you want to say 'to support' someone, you would use apoyar (for emotional or physical support) or mantener (for financial support).

Why is the word for 'this' Este instead of Esto?

Este is the masculine singular demonstrative adjective, used right before a masculine noun like capricho. Esto is the neuter pronoun. It is only used when standing alone to refer to an abstract idea, a general situation, or an unknown object (for example, ¿Qué es esto? meaning 'What is this?'). You never put esto directly before a noun.

Is a capricho always a negative thing?

Not necessarily. While it means 'whim' or 'caprice' (and implies annoyance in this sentence), it is also the standard word for a harmless 'craving,' like wanting a specific snack, or treating yourself to a little luxury just because you feel like it.