You seem to misunderstand the term "power", it doesn't mean it does flat damage. Only a few moves (Dragon Rage (40 damage), Sonicboom (20 damage), etc) does flat damage like that
It is a physical attack meaning your attack stat effect it.
Gotten from Smogon:
Damage Formula = (((((((Level × 2 ÷ 5) + 2) × BasePower × [Sp]Atk ÷ 50) ÷ [Sp]Def) × Mod1) + 2) ×
CH × Mod2 × R ÷ 100) × STAB × Type1 × Type2 × Mod3)
In short: your stats, opponent stats, typing are all factors. The power is only how powerful the move is itself. Pikachu and Raichu for example are the same beside their base stats. Matter of fact Raichu don't even learn moves where Pikachu learn until around Lv50. So using a Thunder Stone too early may screw you up. Let say Pikachu learn Thunderbolt, the attack will do more damage when you evolve him as Raichu have higher stats though the power of the attack itself doesn't change.
To answer your question if Magikarp is worth using. I honestly have no clue. I just couldn't help but to post after I saw that you thought the game work that way. I know he can be good enough to defeat a team of legendary Pokes but I also know there are crazy amount of counters for him like sandstorm.