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: 
There are several usages for katakana.

1. To describe (what feels like) Western origin words
2. To describe onomatopoeia
3. To describe the fact that it is normally written in kanji, but that it is written without it because either the writer wants to write faster, has no access to the kanji form (as in the case where the writer is given the name in a romanized transcription or the writer just heard the name), forgot the kanji form, or does not want to bother to write in kanji for any other reason.

`だめ` has the kanji form `駄目`, and in this case, the reason is likely 3.
There are various reason depending on situation. Yesterday i saw a frog shaped earpick on which it was written カエル instead of かえる. I asked about it and i was told that these things are done so as to avoid any confusion about the product or the instruction given for the user or consumer. かえる can also mean to return , or to change. Also some people can't read Kanji. These are the reason that leads to writing some catchy phrases or words so that there is no ambiguity between both parties. These are informal...

edit: to grab eyes as well...