Space Monsters are actually ridiculously powerful, still.
Their motherships can mass scatter entire gas giants [b]with a near-miss[/b].
Their cruiser class which during the final battle they fielded in the millions or maybe billions are capable of destroying Titan (shown in Diebuster).
Gunbuster just happens to be so powerful it can deflect 20,000 of these attacks by waving its cape.
And it can literally cleave its way through hundreds of thousands of space monsters via a single massively powerful kick.
Granted all of these feats have nothing on Nono literally splitting a black hole larger than the entire Earth in half. It'd literally require energy millions of times more than any other feat in Gunbuster combined (save for the blackhole bomb, ofc).