If I recall correctly "doki doki" is an onomatopoeia for a heart beat in Japan. They're generally written in katakana. Hiragana would just be weird and senseless.
I suggest Kanji instead of Hiragana. In my opinion it's more beautiful, but that's my opinion.
Love (ai) or Heart (kokoro) instead of putt-putt (doki-doki)
Okay, well it's generally written with Katakana. Tattoos are artistic anyway so if you like Hiragana go for it. The Japanese literate will think it's weird, but for everyone else it'll just looks interesting and cool.