It's not "Euro" that the girls were surprised about, but how Mina referred to herself.
Here's the script in Japanese:
Here's the literal translation:
I only have Euros.
Is there something on my face?
(Everyone is laughing)
Here, Mina used わし (washi), which is the first-person pronoun used commonly for elderly, especially from Edo period.
The term is generally only used by the elderly, and its use is often considered stereotypical of them. As such, it is frequently used in TV shows and comics to emphasize the age of characters.
More commonly spelled 私, or in kana to make the reading explicit.
In anime/manga culture, if spoken by young girls, she can be considered as ロリババア (loli baba, old loli): girls who look young but act old. (But since this is a one-off joke from her, Mina is not)