UNDIRECT: October 1987
Little Ghost Q-Taro and Mario appear together in an illustration on the October 1987 issue of this children's magazine. Q-Taro and various characters from Mario and Doraemon are seen running a marathon wearing Mario's hat, with Mario himself waving a white flag.
Since this just shows the characters performing an activity, without much narrative value, it's arguable if it should be considered a link. (see the Cover rule)
We consider it a link because it's a plausible scenario that can be considered to happen in a fictional universe.
Both Super Mario-kun and Little Ghost Q-Taro were published by Shogakukan on the CoroCoro magazine, so they could include references between the two comics.