I like it, or make a rare potion with a 5% maybe scaling increase to luck because there are a ton of towers that can benefit from luck.
I assume that there aren't any smaller potions of luck because it'd be too broken. Consider Beaver's with a 20% chance to stun on hit - stack enough attack speed and they're already more likely to stun. With a bunch of trigger chance potions, they'd be guaranteed to stun each hit. (but that's thinking small, what about stacking luck on the Dark Forge to guarantee an item every round? Or Miss Jilly smiting the rounds etc etc) Which is why I think a purple unique is fine (only one drops) and maybe a rare, as they seem to drop infrequently.
I have a ticket in my backlog which is about luck potions. At least in the ticket description I planned to add common/uncommon/rare luck potions like the other potions. Now, with the possibility to gamble potions, this might be an interesting addition. The luck bonus would as well be quite small on the potions, so players will need to decide wether to spend those potions on the towers or gamble for other kind of potion (crit, dmg).