Hiragana for foreign words: This is against the rule IN GENERAL. In the daily life in Japan however, you may see hiragana for foreign words. One of the reasons may be for advertisement or something. Hiragana for foreign words look strange for us so when we see them, we kind of pay attention to them; this may be the advertisement effect.

In addition, the round structures of hiragana may make people feel softer impression. Ramen is usually written as ラーメン. But if people see らぁめん/らあめん for example, this expression may lead them to imagine that the ramen of the restaurant could be light because ラーメン is usually believed to be heavy. “Soft-light-healthy” could be linked. Also, hiragana is usually used for things which were originally developed in Japan. Based on this idea, らぁめん may lead people to imagine that it is more like the Japanese taste/style.

As for ゼルダ; Zelda as in Cameron’s comment, the SOUND leads to Japanese people to imagine that it came from abroad. Then, expression by katakana also matches with this idea.

It's best to use katakana for foreign words because the Japanese writing system does not use spaces between words. Using all hiragana makes reading a lot more tedious. There are some exceptions however, words sourced from Chinese are often written in Kanji. I have also see raamen spelt both らあめん and ラアメン which I believe indicates if it is a Japanese or Chinese dish. There are also examples of texts written entirely in katakana like Zelda for the Nintendo Famicom because of the space limitations at the time.