It's interesting that you mention the past 20 years, rather then the last 3. Would you say, then, that Trump is not the only person responsible for declining American influence? What other presidential administrations would you hold responsible?
My initial impulse was to even go with 40 years but I thought that I should be gentle and focus more on the worst part of the problem. In my book just about everything since 2000 is just one big pile of mistakes stacked one on another and without Bush and Obama Trump's presidency would be pretty much impossible to ever come to life. He is simply their legacy, there is no doubt about it. In other words all 3 last presidents are/were pissing on public education for the most part and didn't stop "the idiocracy", they exported the concrete industry, they deregulated pretty much everything, they failed in environmental issues, they had too hawkish foreign policy. To me the idea that these 3 people are some kind of great opposites is somewhere between blind and ignorant. Just if the media are pushing that narrative that doesn't make it true.
Now regarding the foreign stuff you are asking me about. Many people are around the world have become sick of USA because this is the culture that claims to be superior to everyone, while in reality often it doesn't have much to show and under Bush that kinda became obvious to "everybody". To the most of us "outsiders" USA has become a country that leads selfish wars that are pushing refugees on our doors, the country which is causing the increased climate related damage within our borders by even denying climate problem, the country that doesn't choose a president by popular vote and lectures everyone on democracy, the country that pushes the idea of healthcare as an industry (at a direct cost of life), the country that thinks that guns in schools are a good idea ... etc. It is hard to pinpoint everything to just 3 men since the system as a whole is just "strange". Obama did somewhat mild out the reputation with his smooth talk but that didn't work in many cases and his foreign policy only added to the mess that Bush did in the Middle East. Plus when he was caught spying EU politicians the best he could do is "oops". Public debt under him completely exploded while at the same time the working class suffered a number of blows due to outsourcing and changes in laws. Plus on his watch the second world managed to do authoritarian comeback, what was probably preventable ... etc.
America simply put itself into the position that people lost pretty much all "enthusiasm" about it due to this "god given" attitude. Which wouldn't even be such a big deal if there were some concrete results to show. You have this story about the land of the free but your prison population is 2 to 3 times higher per capita than in open dictatorships. The greatest military power in the history that fights aliens in the movies and you can't even win Afghanistan for 18 years. The greatest economy in the world (with what is probably the largest public debt in the history of the planet). In my country few decades ago when a average wage was about few hundred $ a month and bombs were falling on the country we managed to have socialized medicine, but the greatest power ever simply can't do it. Or now it is wanted of us to deregulate this system because this is "un-capitalistic", while the fact that we live with much less stress in this regard and we don't have dead untreated people around doesn't seem to count for anything. However this large bag of nothing is now even harming my local pro-western politicians since they have little or nothing to show (and elections are on the door). The polls are showing that as much of 2/3 of people will go rather with ex communists or fascist leaning person over a current pro-American incumbent. One specialized poll even shows that the left voters will rather go with fascist leaning person than with pro-American person as a second choice. Which is because "alternative" people at least don't have a fetish on deregulation, marketizaton and public image .... and that is how "the empire" is melting away all over the map. It is a dream in which most have lost faith, I mean even good chunk of America lost the faith so just imagine how it is outside.
You just can't lecture or even pressure others when your results are complete shit, it simply doesn't stick. Therefore good chunk of the world thinks that USA is mostly a bunch of uneducated people for much longer than "the man himself" announced his campaign a few years ago (if they even think about America at this point). In other words he is just a symptom and a continuation of intellectual bankruptcy trend that started many decades ago.