It would appear that Level 7 isn’t the final level at all. Far from it. I managed to complete the remaining five puzzles on Level 6, then all the Level 7 puzzles, and Mario unlocked Levels 8, 9 and 10 for me. More!
Not just more either – these are 20×20 puzzles (Levels 4, 5, 6 and 7 are 15×15). The first one was clever too – the was a picross of a picross puzzle!