Learn Hardcore Japanese: kono sinseisyo ha, siyakusyo no syorui ni site ha yomi yasui. - This application form is easy to read for a city-hall document.

にしては means for..., considering..., or despite being..., with a nuance of this is a little surprising or better/worse than expected.

So:

市役所の書類にしては読みやすい = It’s easy to read for a city hall document = It’s easier to read than you’d expect from city hall paperwork

The important nuance is that the speaker seems to assume city hall documents are usually not very easy to read.

Why is は used after この申請書?

The は marks この申請書 as the topic of the sentence.

So the structure is basically:

As for this application form, ...
... for a city hall document, it is easy to read.

Japanese often sets up a topic first, then comments on it. Here, the sentence is talking about this application form and saying something about it.

Is the は in にしては the same kind of は as the topic particle?