I remember back when Japan had that big earthquake in 2011, World of Warcraft had a minion you could buy and 100% of the proceeds went to the Red Cross's Japanese relief efforts. So, this type of thing is not unheard of.
The in-game item or reward doesn't need to have anything to do with Ukraine however; in fact, it's probably better if it doesn't and makes sense to show up in the game's world. It can be anything that players will likely want to purchase anyway, but this way they are further incentivized knowing that their money will go to a good cause.
I really don't like this idea, because as long as it's only "a portion" and not 100% of the income then Square Enix would basically be profiting off of death and destruction. Alternatively, I would suggest maybe they temporarily highlight a selection of items from the MogStation and say "for this month, all proceeds from these items go to Ukraine relief." Or -- and I don't know if it's possible -- they could do something like "for this month, the next MogStation item you buy, whatever it is, 100% of the cost goes to charity." There are probably other good ways of doing it as well, but just not anything that involves them skimming off the top of the purchases to make it a profit deal.
I mean, it's not "slacktivism" if it actually results in donating money to a real charity. The idea behind in-game rewards being tied to a charitable donation is it incentivizes more people to give, where perhaps they might not have. I'm sure there are a ton of players who would see a cool glam or a cute minion, and say "OMG I WANT IT" and buy the thing, even if they don't care enough about Ukraine to donate otherwise. Or there's probably people who are thinking "Hmm, maybe I could donate some money to help, maybe I'll do it later, not sure how much, or how to do it... meh, I'll figure it out later," and then never do. But you show them a fancy minion that tells them directly "Here, this is the way. Pay for this thing, we will donate the money for you and you get a fun prize as well as pat yourself on the back for doing the thing. Easy peasy." Call it cynical, call it selfish, I don't care. It works.