So do you approve of using #BlackHolesMatter to protest the heat death of the universe, because it's a much bigger and longer term problem, which affects many more life forms than #BlackLivesMatter?
We have free speech and people can use whatever hashtags they want to. Outside of organizing a terrorist plot or conspiring to kill someone, people have the right to speak and write what they choose.
But to use the line #blackdogsmatter, clearly derived from #blacklivesmatter, is idiotic.