Japanese mimetic words can often be written in either hiragana or katakana. For example, you may see both ふわふわ (fuwafuwa) and フワフワ (fuwafuwa), or どきどき (dokidoki) and ドキドキ (dokidoki). The basic meaning usually stays the same, but the visual impression can feel slightly different.
Hiragana tends to look softer and calmer. Words such as しっとり (shittori), ほっこり (hokkori), and ゆったり (yuttari) often feel more natural in hiragana. Writing them in katakana would still be understandable, but it can make them look a little too strong or simply less natural.
Katakana, on the other hand, often fits words that suggest stronger sounds, impact, hardness, or intensity. ドカン (dokan), ガツン (gatsun), ゴツゴツ (gotsugotsu), and ギラギラ (giragira) all feel quite natural in katakana. Words such as ベタベタ (betabeta) and ギトギト (gitogito) can also look heavier or more unpleasant when written this way.
There is no strict rule, however. Some words are commonly written both ways. きらきら (kirakira) and キラキラ (kirakira) are both perfectly normal, for example. The same word may appear in hiragana in ordinary writing but in katakana in an advertisement, manga, or another context where the writer wants it to stand out more.
Japanese readers are not consciously thinking, “This is in katakana, so it must feel stronger,” every time they see one of these words. Even so, ドキドキ can look slightly more energetic or intense than どきどき, while the hiragana version may feel softer. Years of seeing different styles in manga, anime, advertising, and everyday writing probably help shape that instinctive sense of what looks natural.
So the choice between hiragana and katakana in mimetic words is usually less about grammar than about the feeling of the word and how the writer wants it to look. Native Japanese speakers may not always be able to explain the difference clearly, but they often make the choice quite naturally.
