How casual is too casual for a PhD application personal statement?
That sentence already sounds very flippant. Contrary to its name there is very little that is "personal" about a personal statement (and it's definitely not the kind of soul-searching essay you write for college). It's mostly a document to describe your research experience and how the program fits within your vision of a scientific career. You should certainly demonstrate your interest and passion (in a concrete manner devoid of cliches, of course), but I'd rather err on the side of being too formal than the other way around.