Well seeing that he is involved in highly illegal foreign activity, do I really need to give an explanation for why people don't like him? That's fine, I will anyway. here are some reasons, not in order:
1. He has several sexual assault accusations against him, some are accusations of pedophilia
2. He has proven many, many times that he doesn't understand how the government - or even politics - works, because he is constantly trying to bribe people into supporting him and has asked other world leaders to help him win the election. This, if you are not aware, is completely illegal.
3. He was so quick to insult Obama, especially his healthcare plan, but then failed to come up with a suitable replacement.
4. The wall. This idea that "Mexico is sending it's worst", and that most of the US's problems are - A. immigrants, B. Mexicans - is completely false and pretty darn racist.
5. Even though he never openly said anything in support for hate groups, so, so many of them have endorsed him. Look, chances are that if several white supremacy and other hate groups are supporting you, you're doing something wrong.
6. He is self-centered and bigoted. I know this falls into the opinion category and doesn't give any real actions, it's such a big part of the issue. He insults other people just to get attention and then uses the attention to give himself press coverage.
7. He prioritizes the economy over the environment. I understand that these are two separate issues, but really how important is the economy going to seem in 2050 when so much of the land we take for granted will be underwater and 80% of all other species are extinct? (here are some sources to back up my argument:
https://time.com/5824295/climate-change-future-possibilities/https://time.com/5669022/climate-change-2050/https://www.cnn.com/2019/06/04/health/climate-change-existential-threat-report-intl/index.html)
8. He supports political figures and new media just because they are nice to him. At many press conferences, he let AONN reporters talk, saying that "they were so nice to him the last time", and thus spreading fake news spewed out by AONN. He also was quick to befriend Putin, a dictator who the FBI has called "a lunatic".
9. He doesn't answer questions from left-wing journalists and reporters, but instead says something irrelevant or brings up a completely topic altogether.
10. He is probably the highest elected vector for fake news and false facts that the US has seen. He encouraged people to drink sanitizer when to prevent Coronavirus, and as a result at least 8 people died. He then promised everyone that a drug that was still being tested - one that could (but there was still no scientific evidence saying that it would) cure COVID. Because of this thousands of people with Lupus and other health problems did not have access to their medication.
It's almost like every time he does something that I consider stupid I'm not even surprised any more just because of the past 4 years.
A prison warden must be the very best at kung fu.