Most wallet apps allow you to track your XRP value in a fiat currency. XUMM doesn't.
XUMM focusses at receiving/sending XRP (1 XRP = 1 XRP) and sending/receiving fiat currencies in IOU's (eg. Bitstamp issued USD / Gatehub issued EUR, etc.) Of course the IOU's will represent their fiat value.
We don't show the fiat equivalent of your XRP balance in XUMM, as we don't intend to deliver "portfolio tracker" functionality. Instead, we XUMM will auto-convert your XRP to the right amount of IOU value when signing a payment request, where an IOU is requested and you pay with your XRP balance.