I like the idea of "states being weaker on bosses"! It's basically win-win.
Bosses being immune to status effects, boring, and often makes a character or two useless against them.
Bosses being completely vulnerable to status effects, fun, but too easy, completely shutting down the boss and their strategy.
Also, bosses being vulnerable to status effects but with a REALLY low chance to work is too much of a gamble.
The weaker versions of states for bosses doesn't make bosses boring and immune to everything, while not making them total jokes!
I prefer a different method to this as, to me, it seems to make the status effects equally useless if they do less damage on a boss than they would on a normal enemy. The entire point of most status effects is to be used on enemies that are going to be around for more than 3 turns. If you have a separate but weaker status effect for bosses, then you're basically telling the players not to bother with the status effect on them at all (unless it's something like debuffing a stat or casting silence/reflect).
I've taken a MUCH different approach. My bosses have status RESISTANCES. There are a few that have absolute immunities, but mostly, it's resistances. I know, that seems cliché, but here's basically what I did:
Basic status effects have something from 45% to 55% chance of working against a normal enemy without any kind of weaknesses or resistances. Since my status effects can be "leveled up" over time and as the player progresses, I'm leaving the strategy up to individual tastes. What this does is... It allows you to decide if you want to take out bosses or other enemies with status effects instead of brute force. So, one of the upgrades on another of my status effects might get a boost of 25% to 30% boost in chance of working. If a boss has a resistance to that particular effect (let's say he's got a 50% resistance to it), then with that boost, you've got a much higher chance of getting that effect on the boss. On top of which, my status effects can have their duration lengthened (everything lasts a set amount of turns to avoid fights where getting the effect off once is basically a win as long as you can spam "heal party") or their effects improved. Sometimes those effects debuff the enemy in certain ways.
The real problem with implementing status effects of ANY KIND is the old psychological pain of "is this worth it?". In short, it has to be worth it to a player to spend/waste the turn casting the status effect on the enemy/boss in order to promote the players using it. It's the same issue you run into when you've got bosses completely immune to all status effects. Players naturally relegate those moves to the "useless" bin and never use them in the entirety of your game. If you're making these status effects WEAKER on bosses, you're promoting the same thing with the delusion that you're fixing the issue. If you are going to have two different effects of the same status ailment, then both effects NEED to be worth inflicting, no matter who they are inflicted upon.
Now tell me, if a normal creature has 100Hp and you inflict "Poison" on them so that their health drops by 10% each turn... But you have a boss monster with 2000 HP and that same Poison only drops health by 5% each turn... Do you really think the player would find it worth it to cast? I don't. Especially if I can just cast a magic spell that would hit for anywhere from 6% to %8 damage each turn on that enemy. You've essentially relegated the status effect to "worthlessness" at that point. Especially when you consider that most players wouldn't want to wait around 10 turns to kill a basic enemy and would wipe it out in one or two, and if the status effect isn't useful against a boss (as in, you do more damage in other ways without wasting a turn), there's no real reason to cast it.
My own personal preference is one in which you have no "status effect" solo spells. I prefer it always paired with something. I don't like, "I cast Poison" and then people are poisoned. I like "I cast Spider Vine!" and the spell that does damage for being a spell has a chance to inflict Poison upon the target. That way, if it inflicts the status on the enemy, it's a bonus to casting the spell, and I don't have to worry about wasting a turn trying to inflict a status ailment.