Sterile water would be fine and be better than "mineral water" because we would not know what minerals in what quantities would be in the mineral water.  

Sterile water can be used to prepare F-75 and F-100. However, this is expensive and has no advantage compared to boiled or clean water. Note that powder products, dried skimmed milk, infant formulas, F-75 and F-75, even when industrially prepared, are not sterile.

Some mineral waters can be used for infant feeding and should be all right to prepare F- and F-75. Their mineral content should be checked however. In some countries, for instance in France, strict rules exist to declare that a mineral water is suitable for infant feeding. They must comply with a long list of specifications, including:

Low CO2 (no sparkling water)

Na < 200 mg/L

Sulfate < 140 mg /L

Nitrate < 15 mg/L

Etc…

If you are not sure, better use boiled or clean water.

Besides the answer which has already been given by André Briend I would like to remind you that with each RUTF-sachet about 400ml safe drinking water are required by the recipient. Providing RUTF without assuring sufficient access to drinking water can do harm.