'डेढ़ बजे' is in the oblique case. This is when a noun is the object of a postposition. Though there is no postposition in this sentence, को is implicit (corresponding to 'at' in the English sentence). वह डेढ़ बजे (को) आती है.
The oblique form of बजा is बजे (which is the same as its plural form here). Since times are almost always used in the oblique case, you will see बजे much more than बजा except in sentences such as 'अब डेढ़ बजा है'.