Diamond Tiara seemed to be important for motivation, but I think one of the most important things about cutie marks is that all that ponies need to discover their special talents is themselves, and they would've eventually found it, DT or no.
Now that I think about it though, that's kind of not possible, considering how RD's first Sonic Rainboom was directly responsible for all the Mane 6's cutie marks. I wonder if there's a way that they could've gotten the same cutie marks without the Rainboom....