It doesn't appear that Ketchap is violating a trademark, in that putting a skin on your basketball is not doing the same thing as an actual Pokeball. What they may be doing is passing off, which requires three elements:
- A 'get-up' similar to the registered one (if "you can clearly identify it as a Pokeball", then yes)
- Misrepresentation, specifically that this skin is connected to actual Pokeballs (Usually decided in court, on evidence of witnesses who were/were not confused)
- Damage to trademark (This is enormously subjective, and something that must be considered before suing. If this app is a good advertisement for the original game, there's no damage. If it makes the original look cheap and nasty, or if it could divert customers from the original, there is.)
Intellectual Property law is complex. Do not use this answer as a basis for your assignment, and definitely do not treat it as a legal opinion on the legality of your cool new app.