Sorry, I don't think so. First of all, Diddy has never been in Mario Party, only DK has. In fact, DK is playable again in Mario Party 10.
Second of all, Diddy wasn't in Mario Kart DS either. He's been Mario Kart: Double Dash and Mario Kart Wii. His absence in Mario Kart 8 may indicate they have plans for him in racing games, but Diddy was still in Mario Golf: World Tour, and now M&S Rio Olympics.
Even then, I don't see any problem with Diddy being dropped from the Mario universe. Because it would mean they're trying to separate the DK and Mario universes more.
Wait, he was in Mario Party 9? Heh, interesting, didn't know that.