Not saying that Fiverr should do this without a justified reason that result in a business benefit, showing some empathy and participating in social causes can work to create business relationships and increase Fiverr’s reach, so yes they can have financial benefit at the end of the day by leading this initiative.
Also this can be regulated, by only offering fees reductions to sellers that participate in this initiative, this can be well administrated with the tools they have so this initiative works in the expected way and preventing people taking advantage for a selfish purpose. I’m not sure what holiday situation are you referring too, I have not seen anything similar to what I’m proposing here so far, well not in Fiverr.
Actually this is not an original idea, I saw that other platforms are doing this, e.g. Codementor, basically they are recruiting mentors (sellers) that want to donate time or offer discounts, so if others are doing this there should be a good strategy behind, and I also think they are considering the risks you are mentioning, however, I strongly believe this kind of acts are an investment and can result in bigger benefits in the long run.
In other words, this race has already started, doesn’t matter if Fiverr wants to participate or not, others will do and will be remembered by that.