It’s safe, sure, but it’s not exactly healthy. To pikachu, ketchup is essentially candy. It’s not particularly nutritious, and it’s possible to have too much—which, incidentally, would result in more of a stomachache than anything dangerous. Just be sure pikachu don’t get access to more than one bottle, and your customers (and your budget, for that matter) should be fine.
Additionally, yes, any pokémon can have diabetes, including this kirlia. However, most pokémon should actually avoid sugar either way (as too much sugar can result in a whole host of complications other than diabetes, including kidney issues), so it would indeed be worth it to introduce reduced sugar or sugar-free options on your menu. If it helps, to a pokémon, sweet berries are about as effective of a substitute sweetener as one would think.