Apple’s fiscal year starts in October, so Q1 sales account for the holiday season.
Are iPad Sales Declining?
In a word: Yes.

Matt Buchanan / Flickr / Wikimedia Commons
But this is to be expected.
The upgrade cycle is what drives the industry.
Through 2018, Apple reported an average of 10 million units sold per quarter.
Matt Buchanan / Flickr / Wikimedia Commons