Mi madre dice que la piña con yogur está tan buena como las cerezas con miel.

In normal Spanish, possessives like mi, tu, su, nuestro usually replace the article.

So you say:

• mi madre = my mother
• tu hermano = your brother
• su casa = his/her/their house

Using la mi madre would sound wrong in standard modern Spanish.

Here que introduces the clause that follows dice. It works like that in English:

• Mi madre dice que... = My mother says that...

In English, that is often optional, but in Spanish que is normally kept in this structure.

Spanish often uses estar for how food tastes or seems at a particular moment.

• Está buena = it tastes good / it is delicious
• Es buena = it is good in a more general sense, such as good quality, healthy, or beneficial

So here está is the natural choice because your mother is commenting on the taste of the food.

Because buena is an adjective describing the food, and it agrees with la piña.

• La piña está buena = The pineapple tastes good
• La piña está bien = The pineapple is fine / okay

Bien is usually an adverb, not the word Spanish normally uses for food tasting good. For food, Spanish commonly uses bueno/a.

Because buena agrees with la piña, which is feminine singular.

• la piña → feminine singular
• therefore → buena

If the subject were masculine, you would use bueno:

• El yogur está bueno

Because buena agrees with the first thing being described: la piña con yogur.

The structure is:

• la piña con yogur = subject
• está tan buena como las cerezas con miel

So the adjective matches la piña, not las cerezas.

You can think of the full meaning as:

• La piña con yogur está tan buena como las cerezas con miel están buenas

But in Spanish, the second están buenas is omitted because it is understood.

Tan ... como means as ... as.

So:

• tan buena como = as good/tasty as

This is the normal structure for comparisons of equality with adjectives and adverbs:

• tan alto como = as tall as
• tan rápido como = as fast as
• tan buena como = as good as

Because Spanish often leaves out the article after con when talking about ingredients or substances in a general way.

So:

• piña con yogur
• cerezas con miel

means pineapple with yogurt, cherries with honey.

If you were talking about a specific yogurt or a specific honey, the article could appear:

• con el yogur griego que compré
• con la miel de esa tienda

Yes. Con suggests a combination: pineapple served with yogurt, mixed with yogurt, or eaten together with yogurt.

If you wanted to mention them as two separate items, you would more likely say:

• la piña y el yogur

So con gives the idea of one food combination.

That is just the natural way these foods are often referred to.

• la piña can refer to pineapple as a fruit or food item in the singular
• las cerezas is often plural because cherries are usually thought of as several individual fruits

So this is not a special grammar rule in the sentence; it is just natural usage.

Yes. Yogur is a standard Spanish form and is very common in Spain.

You may also see yogurt sometimes, especially on packaging or in international contexts, but yogur is fully normal Spanish.

Yes. With food, estar bueno/a usually means to taste good.

But with people, especially in Spain, estar bueno or estar buena can mean to be very attractive / sexy.

So context matters:

• La piña está buena = The pineapple is tasty
• Ese chico está bueno = That guy is hot

In your sentence, because the subject is food, the meaning is clearly about taste.

Yes, rica would also be possible:

• La piña con yogur está tan rica como las cerezas con miel

Both buena and rica can describe tasty food. In Spain, estar bueno/a is very common and natural for food, so the sentence sounds perfectly normal as it is.