If you mean 'Can I use 2* Mint to increase her 5* ver. skill lv?' Then no, you can't. To increase skill lv. you need the girl with EXACTLY same skill. They don't need to be the same girl, or same rarity. Just same skill name and you're good. Although all 5* and 6* skill are all unique. So you can only use the same girl or skillbloom to increase their skill lv.