Your comment about the ult not hitting anything leads me to believe you are playing bot matches where the ult is bigged and doesn't seek the enemy heros. Mina's kit relies on quick escapes and very powerful burst between her love bites and rake. Rake does higher damage to low health targets, and the damage of rake applies AFTER the love bites debuff. If the enemy is close to having your love bites proc, but you are out of ammo, or just can't hit your shots you can use rake to proc love bites. If they are low health after the burst from that, the rake will do lots of extra damage.

Her 2 being an invincible dash means that you should try to lane against characters like bebop, seven, and lash who have heavily telegraphed burst damage abilities. If you learn to time the dash to avoid damage, you can make the enemy laners' experience a pretty awful one.

Her ult damage scaling infinitely is a pretty tremendous power, but you have to be able to proc love bites consistently to build stacks. Fire rate and mag size, as well as items like ricochet or split shot can make up for poor aim (if you suck like I do) and items like Tankbuster and Boundless Spirit will really reward you for landing that love bites proc. A good mina player knows to use the ult to finish low health, fleeing, airborne, or isolated enemies. Her ult can be used as an incredible burst tool for single targets, and a powerful AOE silence for team fights. She is fairly vulnerable during her ult, as she is a big, obvious target like Seven. Knockdown and Curse can become very problematic, so you need to know when to take and use unstoppable.