Depends on the skill level of Diddy Kong.
While a do agree with CBK above, chances of getting owned even by a GOOD diddy kong are still high if you throw the nanners BACK AT HIM. Master diddy's know like 13+ ways of recatching thier stuff. As a person who's faced not just one but MANY diddy's and Toon Links of varying skill levels, the best bet for me is to continuously throw the nanners UP. That way, the bananas will take EONS to dissappear cuz they are not grounded, and since they are your possession, they'll counter anything diddy does. Depending on the screen and stage, the bananas cannot be seen until its too late (its so much harder to telegraph and catch an item successfully by MOST people if the banana is falling down OFFSCREEN and a intense fight is going on). Also, for Pikachu a full hope retreating d-air or f-air or b-air is quite safe (even again diddy's f-air) if you space right, and pika catches the falling banana safely (since it is your possession, you don't have to worry about perfectly sweetspotting the regrab without nanner hitting you). They will HAVE to wait LONGER until the nanner disappear (t-jolt spam them while catching and throwing nanners up ^_^), or melee you without nanner support, or risk being ABOVE you in order to reclaim banana support (free licking ^_^).
Next to the stategy above, if the diddy rushes foward, shielding the first nanner and jump-n-air out-of-shield (while catching nanner) is the next successful thing i can do. Pika can also t-jolt or QaC on the way back down if the situation calls for it while holding the banana.
Trading trips with T-jolts are sometimes good too. Since most diddy's shuffle and throw forwards anyways, correct me if i am wrong but nanner damage is less than t-jolt damage i believe. If you trade a trip and LAND the t-jolt, you can get out of trip and have damage lead in every case.
That's what i think off the top of my head. Also thunderboom is good against diddy's so long as you are NOT under them (d-air...), since side-b even if it grabs you, the rod part with force diddy to release you. If the boom happens then thats one stock almost immediately gone.