sono bousi ha kanozyo ni yoku niau si, zimina kooto ni mo pittari da.

その means that, so その帽子 is that hat. It points to a specific hat that is already known from the situation or previous conversation.

Using その instead of leaving it out makes the reference more specific:

• 帽子は = as for hats / as for the hat
• その帽子は = as for that hat

So その helps identify exactly which hat is being talked about.

は marks 帽子 as the topic of the sentence: as for that hat...

The sentence is basically describing that hat in two ways:

• it suits her well
• it also goes perfectly with a plain coat

So 帽子は sets up the thing being discussed, and the rest of the sentence tells us things about it.

With 似合う, the thing or person that something suits is usually marked by に.

The pattern is:

A は B に 似合う
= A suits B
= A looks good on B

So here:

• その帽子は = as for that hat
• 彼女に = on her / for her
• よく似合う = suits well

In other words, that hat looks good on her.

No. Here よく means well, not often.

That is a very common learner question because よく can mean either:

• often
• well

In this sentence, よく似合う means suits well or looks very good on.

So this is about degree/quality, not frequency.

似合う means to suit, to look good on, or to be becoming on someone.

It is often used for things like:

• clothes
• hairstyles
• colors
• accessories

Examples:

• この色は彼に似合う。 = This color suits him.
• そのドレスはあなたによく似合う。 = That dress looks great on you.

In this sentence, 似合う is saying that the hat matches her appearance well.

し connects one point to another. A natural English equivalent here is and, plus, or and besides.

So:

• 彼女によく似合うし、地味なコートにもぴったりだ。 means something like:
• It looks good on her, and it also goes perfectly with a plain coat.

A useful nuance of し is that it often sounds like the speaker is listing reasons or good points, sometimes with the feeling that there could be more.

So it is not just a mechanical connector. It can give a sense like:

• it suits her well, and on top of that, it matches a plain coat too

地味 is a na-adjective.

That means:

• before a noun, it takes な
• at the end of a sentence, it can be followed by だ / です

So:

• 地味なコート = a plain/subdued coat
• このコートは地味だ = this coat is plain/subdued

地味 usually means something like:

• plain
• subdued
• not flashy
• conservative in appearance

So 地味なコート is a coat with an understated look.

にも is just:

• に
  • も

Here:

• に marks what something is a good match for
• も adds the meaning of also / too

So 地味なコートにもぴったりだ means:

• it is also perfect for a plain coat
• more naturally: it also goes perfectly with a plain coat

The も shows this is an additional good point after the first clause.

ぴったり means perfectly, exactly right, or a perfect match.

It can be used for:

• physical fit: a shirt fits perfectly
• timing: arriving at exactly the right time
• matching: colors or clothes go perfectly together

In this sentence, it means the hat is a perfect match for a plain coat.

So 地味なコートにもぴったりだ is not about size; it is about style matching.

Because Japanese often leaves out words that are already understood from context.

The topic その帽子は carries over into the second part. So the full idea is:

• その帽子は彼女によく似合うし、[その帽子は] 地味なコートにもぴったりだ。

In English, we usually repeat the subject:

• That hat looks good on her, and it also goes perfectly with a plain coat.

In Japanese, once the topic is clear, repeating it can sound unnecessary.

だ is the plain-form copula, like a casual version of is.

Here, ぴったりだ means is a perfect match.

The sentence is written in plain style:

• 似合う
• ぴったりだ

A more polite version would be:

• その帽子は彼女によく似合うし、地味なコートにもぴったりです。

So だ does not change the basic meaning; it just makes the sentence sound plain/casual rather than polite.

It is saying both.

The sentence gives two separate positive points about the same hat:

• 彼女によく似合う = it looks good on her
• 地味なコートにもぴったりだ = it also matches a plain coat perfectly

So the hat is being praised from two angles:

• it suits the person wearing it
• it coordinates well with another item of clothing

That is exactly why し works well here: it links two favorable points together.