Yeah, the power of your attack depend of how much the power value is raised.
If you keep attacking without raising it, your character will be slower and his attack power will be greatly reduced.
By the way, I forgot to include it in the readme but Zsnes 1.51 (dunno about older versions) doesn't emulate the game properly (though it's playable). In the second half of the game, the characters portrait often glitch and to fix it, it require to go in the option menu (START) then leave it. I guess it's a VRAM emulation problem (as it also happen with the japanese version and when I was young, I had the original game and never got this problem).
So yeah, I recommend to use other emulators (Higan/Bsnes, and maybe Snes9x, though I haven't fully tested the game on this last one).
Thanks, just noticed a typo thanks to your screenshot! It should be "Urgh!" and not "Ugh!"