Learn Spanish (Latin America): Esa silla es cómoda, pero su diseño es peor. - That chair is comfortable, but its design is worse.

Just like in English we say "worse" instead of "more bad", Spanish has irregular words for comparing things. Peor means "worse". You should never put más in front of it, because peor already includes the idea of "more".

Does peor have a feminine or plural form?

It does not change for gender; you use peor for both masculine and feminine things. However, it does have a plural form: peores. If you were talking about multiple designs, you would say sus diseños son peores.

Why use esa instead of eso for "that"?

Since "chair" (silla) is a feminine noun, we have to use the feminine word for "that", which is esa. Eso is a neutral word used for abstract ideas, situations, or unknown objects, but it is never used right next to a specific noun like silla.

Why do we use es here instead of está?

We use es (from the verb ser) because being comfortable and having a bad design are considered permanent, built-in characteristics of this specific chair. If you used está, it would usually describe a temporary state, like a person currently feeling comfortable while sitting.