AN is right, but it's also fun to think about why some people do have a big negative reaction and aren't just performative. I'm going to guess it's mostly Americans freaking out - I don't think many other places use ketchup enough to care, and Americans are an awful lot of the internet. I'll use the opportunity for some fun cultural commentary for our non-American readers.
Ketchup is in a funny place in America. It's extremely standard and respected on a limited, defined selection of things - burgers, fries, hot dogs, chicken, and that's probably it. It's generally seen quite negatively outside that. Like ketchup on eggs is definitely a thing, and some people love it, but most people will turn their noses up at it.
Even further, ketchup has a reputation / stereotype as something that people overload on food when:
1) The food is exotic / unfamiliar to them and they actively don't like that, or they just habitually have a very limited American-focused diet, such that they're drenching an American-associated condiment without regard for how its flavor will match the food's flavors;
2) They don't like the food, masking its flavor with this strong, establishment consistent flavor. Often overlaps with 1.
3) They have poor culinary taste, carelessly dumping this strong flavor, which Americans generally believe is only compatible with a few things, onto whatever else. Often overlaps with 1, again.
It's obviously good to try new things, and not ideal if someone refuses that in favor of dumping ketchup on everything, but there's definitely a classist angle here too. Poorer, rural, white, Appalachian/Southern people are stereotyped as having much more limited, boring, processed, cheap, unhealthy, etc. food. A lot of white bread, heavily processed meats and other products like bologne and Cheez-Whiz, and yes, ketchup. Which is how you get things like ketchup sandwiches, a stereotypical poor white person food.
Soapbox time: I'll emphasize "cheap." This food often is easy to buy and cheap in America and is calorie-dense, and there is some truth to people in these groups eating it. However, it is better to understand the poor-health aspect as coming from poor wealth, food access, and knowledge / guidance, versus people being specifically flawed or uncaring when it comes to health.
Also. While of course dietary health is good, and culinary expertise and palate are skills that can be cultivated, I am hardly here to trash on any group's cuisine. We don't expect our friends to become food critics, and we usually aren't close to that level ourselves, so we shouldn't expect them to think and choose food like food critics, or seriously trash them for making choices we don't like. Friendly ribbing is different of course. I'm not saying anyone trashed anyone here, just a stance I have when it comes to food.
OK. We were talking about pizza. The average American would interpret "ketchup on pizza" as a direct squeeze on a cooked pizza from a bottle, and they'd have an immediate negative reaction from what I talked about, not even thinking about the specific taste combination. I wouldn't personally be a fan of it myself - not only am I very finicky when it comes to food in general, but the lukewarm temperature of the ketchup with the hot temperature of the cooked pizza is not a dynamic I like. Of course, not only might you like it, you may also do different things with the ketchup than the average American would expect from hearing "ketchup on pizza," like a different or less standardized ketchup flavor, or even putting it on before you bake the thing. Heck if I know. Have fun