Tbh, im kinda in a nsmbds phase and i thought what better way to try and experience it then trying to change the body model because why not. I saw a post saying i needed some old programs like 3DS max or Maya. But could it also be possible with blender? If anyone could help me i would appreciate it. (if i did something wrong then thats becuase i LITERALLY made this account today)

You should be able to with some optimization, i would recommend finding a way to turn the NSMB-DS Mario/Luigi model into a .dae/.obj file and working off of that.