It is permissible to fast in Mina if there is no hardship because intending to make things difficult is forbidden and Allah has given the excuse and the permission for travelers not to fast Ramadan which is mandatory.
But if there were no difficulties at all, it is permissible to fast, but not to fast Arafah whilst in Hajj, as our Prophet salla Allahu alaihi wa sallam didn’t fast it.