I remember how before the monthly ranks were introduced, you would obtain a serverwide multiplier upon buying obsidian rank. The thank system is similar to the one you propose. I don't know the exact rewards of a thanks, but it was something like 100XP and 20 point per thanks.
Let's now say, you would activate that multiplier on peak hours right now, reaching maybe a thousand members online, and let's say all of those people also know how to thank and manage to do so, you would get 100.000 XP and 20.000 points.
I remember multipliers were popular before the personal multipliers became a thing. Right now, they are not even in the shop anymore. It would give a different reward depending on when you activate this. How I see this happening in reality:
I want to buy a rank, but I don't want to waste this 10GBP, so I wait for the perfect moment (1k people online). This moment doesn't come, however, which I then decide it might not be worth it to buy the rank at all. But, let's say this is all completely false and people would be stimulated to buy a rank for this: people would be annoyed by the amount of spam of these messages, as it would make sense for people to do it at the same time, which is the time people are most active on. Not only does this influence gameplay, but also the queue time, as a lot of people will get online and AFK for half an hour, just for the possible messages.
The point you make is negated as a whole already. You get almost more XP from winning one SkyWars game compared to an average amoung of people online that would thank you for a thing like this. So, XP rewards must be higher, but that brings a ton of problems like discussed above, but in addition it also would screw up dev time for people that would stop with subscribing to a rank every month and subscribe again next time for extra rewards.
Overall, I think this idea is complicated, brings a lot of negatives and eventually is just a waste of dev time.