I was thinking about that, it's actually a pain.
We can aggregate data, but we loose the signature, so it's useless. Perhaps one solution is to somehow use a Merkle tree structure?
Or maybe reactions are not so important.
Discussion
As an aside, I get that signatures prevent relay manipulation of the data; but, what prevents client manipulation of what is presented to the user?
Maybe aggregated data is good enough if the underlying source reaction notes remain available to be spot-checked for signature verification.