I have replaced all DK's portraits (and sprites) and i use his slot with the custom character i'm making.
For some reason, now i'm getting a game crash too when i finish a battle (or a V.S. race), instead of the Mario portraits. Crashes even if you don't select the custom character in game.
In Grand Prix mode i still get the Mario portrait instead:
I get that also with the normal characters.
So, basically, at the end of every mode i always get the Mario portrait for every character (or a game crash). But i noticed a little difference: if i use the custom character, it does the Mario portrait victory animation for two times then it switches always in his "loss" frame (like the image above). And there is that "sherbet land" title image behind him. Instead, with normal characters, the Mario portrait acts normally.
Ah, i didn't touch anything over DK's portraits/sprites.
EDIT: i just tested, this issue is valid for every Custom Character. Even if you replace one single frame of portrait, you'll get these crashes/mario portraits at the end of GP, battle and VS modes.