Thats not quite right, there is lots of techniques to get more points, and that varies from character to character. For example, with raiden it is possible to combine torpedos with other attacks to finish the round very quickly and earning more points from the time bonus that in the end you end up earning more points than if you would finish the round only using the knee kick (wich is very time consuming). Raiden has 3 attacks that values 5,000 (knee , throw and torpedo) and scorpion has only 2 (knee, throw). Jonny has shadow kick that also values 5,000 and also is possilble to make powerfull combos so,you finish quickly and earns lots of 5,000s from the shadows and also ean lots of extra points as time bonus. With raiden there is also some techniques that you can win the round and still throw an extra 2 torpedos followed by a hell fast teleport+fatalitty. On the other hand, scorpion has the harpoon+knee combo that combined values 7,000 and does less damage so you can throw lots of theae to get more points, specially on the leeching round ( the round that you lose). Raiden, in lots of specific situations ,has also the combination knee+torpedo that toghether values 10,000 and is very fast... There so much possible combos to earn the fight quickly and at the same time trhows lots of 5000 poitns hits and in the end earn more points from time bonus. Thre is also some situations that i can lock the oponnent in the side of the screen, give a knee followed by a trhow to the other side followed by a rapid torpedo, this would earn 15,000 points in less than 4 seconds. There is much more... If i would write all the possible combinations here you would spent hours reading this. The key to maximize score is to throw specific fast and high valuable combos at the right times to finish the round as soon as possible andbstill get more points from time bonus. Dont forget that each second you lost you lose 1,000 points.
About score system i doubt that score changes for each characters except Sonia (with Sonya is not possible play against Reptile). Because each character have the same pattern to play for max score possibile: only close high kick for all time + last hit with throw + fatality or something similar. Work with all chara. So I can say that if you made 25m with Raiden you can obtain the same score also with Sub-zero or Scorpion etc.
Mortal kombat has lots of powerfull combos and possiblities, thats why this game is so amazing...