My goodness, isn’t that cheerful!
can you use the back of the fabric, so it isn’t so bright?
How about tea dyeing it, so there’s less contrast?
somebody else suggested color blocking; could you use a solid color fabric for most of the shirt, and use this for trim, like a yoke and pockets?
I think a very simple, tailored style would reduce the childish effect of the print, maybe a fitted tank top or something with simple narrow sleeves–nothing full or flowy; minimize the amount of fabric used. I wouldn’t add ruffles or daisy buttons (available right now at walmart!).
How dressy is the event? It really would fit right in at a picnic, but it’s got a long hill to climb to look right at a formal wedding.
Indoors? Outdoors? you could add a sweater or vest, or use it as the lining of a vest.
Maybe layer it under a plain white blouse…
just thinking out loud, maybe something will inspire you.