Learn Polish: Taki niestosowny strój to po prostu brak manier. - Such an inappropriate attire is simply a lack of manners.

Niestosowny is the adjective form for something inappropriate or unsuitable. If an outfit is niestosowny, it means nie wypada (it is socially unacceptable) to wear it. You could express the exact same idea by saying: Nie wypada nosić takiego stroju (It is inappropriate to wear such an outfit).

Why do we use to here instead of jest for the word 'is'?

This is a great place for a quick refresher on the Polish noun-equals-noun structure. When linking two noun phrases, using to lets you keep both in the nominative case (strój to brak). If you used jest, the second part would need the instrumental case: Taki strój jest po prostu brakiem manier.

Why is it brak manier and what case is being used?

The noun brak (lack of) always forces the noun that follows it into the genitive case. Here we need the genitive plural, because we are talking about manners in general. Manier is the genitive plural form of the feminine noun maniera.

Could I just use ubranie instead of strój?