Meaning of 嘛 in this context?
嘛 has 2 functions.

The first translates as showing something's obvious, much like an "of course" would on the end of sentences in English. An example would be 「你的妹妹很漂亮嘛」if you're taking it as obvious their little sister is pretty (I assume you're using 繁字體). Clearly not the case here, as your example is in the middle of a sentence.

The second function is to signal a topic for emphasis. For example


  大熊貓嘛，孩子們都很喜歓。Pandas ah, children all like them"


嘛 is used to provide emphasis. In this case, "台灣都嘛" emphasizes that "As for Taiwan, it makes nothing but low cost movies about the humanities". It's not a perfect translation, but as with all particles, finding an English equivalent isn't always easy, or possible. If you know any Japanese, the particle は can be used for similar effect in many sentences.

Most importantly, 嘛 is emphasizing 台灣都. It marks it out as a topic, and is a small break in the sentence, like a comma in English. You could take your example sentence as


  台灣都嘛、是做小成本人文電影


which we can take in English as along the lines of "Taiwan, it just makes low cost humanities movies" or maybe better "Ah, Taiwan, it just makes low cost humanities movies"