There's a lot of yuck to write about Yoshi (tbf Mario has yuck to show off too but only because he's so prominent) but the official material for Mario Party Advance shows Yoshi's solo art without his orange spines.
If you have any doubts that maybe it's just some bad editing, look at the key art
If you're like "holy crap I remember that art having spines"
Me: A crossover between Super Mario 64 and Donkey Kong Country would be so awesome! I can't wait to imagine Mario traversing 3D versions of levels such as Jungle Japes, Bramble Blast, and Gang-Plank Galleon. What could possibly go wrong?!
The end result:
(A haphazardly put together poster featuring just two pieces of Mario stock art, which makes the poster focus more on the DKC characters as a result.)
so many whippersnappers on cranky's lawn, including mario himself, that cranky had a heart attack and had to be rushed to the Kong Klinic. this is why he's not on the poster.