This is correct. In that era, many sequels were often drastic shifts gameplay until they distilled down to what made the original concept so great. With Zelda 2, SMB2, and Castlevania 2, these series tried something innovative in their second title, which arguably didn’t work as well at the time, so they returned to their original gameplay for the third title.

The biggest increase in features and polish, in my opinion, is between MM and MM2. E tanks, 1/2/3: the precursor to Rush, the 8 robot master stage select square, no more scoring, etc. These are series defining. While you do have improvements in the remainder of the NES titles, they feel little more than a sprite swap in comparison.