Could someone tell me if the phrase 'mono no aware' should be written in a certain way for example hiragana/kanji and if so why? Could you please show me the correct way it should look? Thanks
もののあわれ is the modern way of writing it.
もののあはれ is the old way of writing it if you want an old flavor to it. It's a poetic word, so we like to see it this way, too.
It could be 物の哀れ, but I personally find 物 doesn't look very good; this kanji letter has a feel of breaking the poetic mood to me. :D I prefer ものの哀れ, but I think the hiragana versions are more common and look friendlier.