Normally, I would actually agree with the point you're making here. It usually takes a couple of years for Presidential policy decisions to work their way through the economy - tax changes phase in, subsidies need time for people/companies to take advantage of them, etc. Often the results aren't seen until the next administration.
However, in some cases, results of some decisions can be seen immediately, and that's what has happened over the past 15 months. If you don't want to be blamed for the economy tanking while you're in office you probably shouldn't do things that will immediately harm the economy. Like start an unnecessary war of choice that literally anyone with a passing understanding of the region would be able to tell you will cause oil prices to skyrocket, or put massive tariffs that will cause prices to rise on almost everything.
Everyone can look and see "Gas prices were X before this war, now they're way, way higher." So Trump going to get blamed for that because it's cause and effect. If he had done the work BEFORE starting the war of convincing the country it was needed, he might have gotten some slack, but he didn't do that. So, all most people see is "we started a war for no reason and now gas prices are high."
The irony is all Trump had to do was not do anything drastic and the economy that had been improving would have continued to do so, probably pretty significantly, he could have taken all the credit - and he would have gotten it from almost everyone outside of hardcore partisan Democrats.
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