I'd say Mario vs. Donkey Kong qualifies as a platformer, even if it's somewhat more puzzle-oriented and less action-oriented than most other Mario games. As for competitive value, the scores are largely based on your time bonus, which is determined by a timer that counts in seconds. There's bound to be a fair number of record ties, but I think "easy records" will be somewhat rare. The best strategies are usually not obvious, and in many levels it's quite a challenge to cut that last second needed to tie the record.
Alternatively, you can look at a few
videos to get a better idea about this.
Mario vs. Donkey Kong 2 deviates more from the typical platformer: you use the DS's stylus to direct Mini-Mario toys around obstacles and to the goal, without controlling anything directly. I haven't played this game as much as the original, but I think it's definitely competition worthy. It's just that around World 3 I stopped trying to form optimal strategies for high scores, since the levels were getting so complicated.
Also, Yoshi's Island DS has quite a nice Time Trial mode. There may be a difficulty with adding it though: since you earn time deductions for the stars, red coins, and flowers that you finish with, times can easily go into the negatives (my time for the first level is something like -1:16.xx). I'm not sure if this could be implemented in the rankings.
