Not really that unlucky yet. You're still not behind the probability curve. Statistically, you have a 1/9 chance of getting Meowmere per Moon Lord kill.
Realistically, it could take way more than nine due to the Birthday problem.
Ultimately, the only real advice is to just keep grinding out kills. Don't worry about item drop rates. Treat it like a 50/50 drop: it either drops or it doesn't.
That's a gaming superstition known as the Desire Sensor. When you're actively looking for an item you'll focus more and more on the fact you don't have the item yet until it eventually eats you alive.Yep, for some reason I feel like whenever you're looking for a specific item all the other ones tend to drop. Might just be me... or the game can read minds!
Just really unlucky, unless all bosses are like that. I had a similar experience with the golem, it took me 40 kills to get the picksaw.