Question: I noticed that "dame", which means "not good" or "don't do that", is sometimes written in manga as katakana. I was wondering, is it because katakana is used to express a strong feeling?

Sometimes, there are also other words written in katakana. In general, what are possible reasons why people would choose *katakana* for a word instead of *hiragana* or *kanji*, especially when that word is not usually spelled with *katakana*?

Answer: 
In addition to user458s list:  

* To give visual and/or *very* slight semantic emphasis. Almost like using **bold** or *italics* in English.

> 
> それはだめだよ！
> 
> 
> それはダメだよ！
> 
> 
> 

You can see that the latter stands out more. As for the onomatopoeia, those are often a little emphasized too, so it probably overlaps with reason 2 a good amount.  

* Plant, animal, and mineral names are often written in katakana.
Japanese often use katakana for certain Japanese-origin words when they are difficult to write or imagine in kanji. I guess it's because they sound somewhat like 'foreign' or 'onomatopoeia' to Japanese.

Common examples I can think of are:

```
滅茶苦茶 as メチャクチャ
御洒落 as オシャレ
出鱈目 as デタラメ
駄洒落 as ダジャレ
辻褄 as ツジツマ
我儘 as ワガママ
馬鹿 as バカ

```

Of course these are not foreign words, and even elementary school children understand their meanings. However, they are so difficult or bothering to write in kanji that it became very common to write them down either in hiragana or katakana, even for adults using word processors. Daring to use kanji may look as if you were an old man.

Whether to use hiragana or katakana is up to the writer's taste. I don't believe it has something to do with emphasizing. Generally, using hiragana tends to give soft, childish, cute or girly impressions, although the nuance is fairly subtle.