Is it okay to make dua for becoming a billionaire?

There's no issue with wanting to have more money. Some of the companions were amongst the richest, Abu Bakr as-Siddiq, Abdur Rahman ibn Uth for example, Uthman ibn Affan. But it's how you use your money. So that's the key aspect. Because we know on the Day of Judgment, the Prophet sallallahu alayhi wa sallam said, in the hadith collected by Matir Mithi: Our feet will not move until we have been asked about certain things. One of them is how we attained our money. And one of them is what we used our money for.