Learn Hardcore Portuguese (Portugal): Eu troco de roupa e visto o pijama mal chego a casa. - I change clothes and put on my pyjamas as soon as I get home.

Portuguese uses the pattern trocar de + clothing item to mean “change clothes.” The de shows you’re swapping one garment for another. Because this is a general action (“change clothes”), the noun stays indefinite, so no article is needed. Saying troco roupa would imply you’re exchanging clothes with someone, and troco da roupa wouldn’t convey the intended meaning.

What’s the difference between visto o pijama and visto-me? Why isn’t there a clitic pronoun here?

Portuguese has two patterns:
1) vestir + direct object (transitive) – e.g. visto o pijama (“I put on my pajamas”).
2) vestir-se (reflexive) with no object – e.g. visto-me (“I dress myself”).
You cannot mix both (visto-me o pijama is incorrect).

Why is there a definite article o before pijama?

In Portuguese, we typically use the definite article before clothing items, body parts and similar nouns when speaking generally. So o pijama is “the pajamas” you put on, even if it’s just “my pajamas.” Omitting the article would sound odd in this context.

What does mal mean in mal chego a casa? It can’t mean “badly,” right?

Correct: here mal is an adverb meaning “as soon as.” So mal chego a casa = “as soon as I arrive home.” It has nothing to do with “badly” in this sentence.

Can mal appear before the verb, like Mal chego a casa, troco de roupa?

Yes. You can place mal either before or after the verb in colloquial speech, but in European Portuguese it’s more common at the start:
Mal chego a casa, troco de roupa e visto o pijama.
Both orders convey the same “as soon as” meaning.

Why do we say chego a casa and not chego em casa?

The standard norm in Portugal uses chegar a + place, so chegar a casa, chegar à escola, etc. Saying chegar em casa is more typical of Brazilian Portuguese. In European Portuguese, you should stick to a.

Why does the sentence use the simple present tense (troco, visto, chego) instead of a past or continuous tense?

In Portuguese, the simple present is often used for habitual actions or sequences: what you regularly do. Here it describes your routine immediately after arriving, so the present tense is perfect:
“Every time I get home, I change clothes and put on my pajamas.”

Is the subject pronoun eu necessary? Can we drop it?

Portuguese is a pro-drop language—verb endings indicate the subject person. You can omit eu, especially in informal speech:
Troco de roupa e visto o pijama mal chego a casa.
Including eu is optional and adds emphasis or clarity.

Could I replace mal with assim que or logo que?

Yes. Both assim que and logo que mean “as soon as” and can substitute mal:
Assim que chego a casa... or Logo que chego a casa...
Your choice depends on style (both are perfectly correct).