It depends what you mean by mixing. Normally, Japanese switches freely back and forth between scripts from one word to the next, so you can have katakana, hiragana, romaji and kanji all in the same sentence. But you generally don’t alternate between hiragana and katakana in the same word. Kana with kanji happens sometimes, when one of the characters in a kanji compound is uncommon, it may be written out in hiragana or katakana instead. Sometimes one character is written in hiragana and the other in katakana.
