I assume you are referring to Section 4, which provides for the president to be replaced if incapable. The point of it is so the USA can replace a president who has gone gaga. Naturally it leaves it up to the named officers to decide that as it is impossible to prescribe the necessary circumstances in law. Unfortunately, calling yourself the king of the Jews or trying to buy land that is not for sale is not loony enough. Trump just shoots his big mouth off too much.
But you need it there in case of real dementia. Here in the UK, the equivalent is the Regency Acts, We've had regencies in the past in the case of underage kings or insane kings but those were arranged as and when. Americans will remember George III - he eventually went mentally ill in the early 1800s so his son took over as Prince Regent in 1811 and became George IV when he died. OK, but ideally you want a law about what should happen so when in 1936, Edward VIII's abdication led to the heir to the throne being a 10 year old girl, Parliament enacted the Regency Act 1937. (You know who the girl is - she's now Queen Elizabeth II.)
What that does is provide for 3 out of 5 top people (including the 2 top English judges) to declare a regency in case of the monarch becoming incapable. The reason I'm telling you this is the Act requires them to do it on the basis of medical advice. That is a glaring omission from the 25th Amendment. And medical advice would say that Trump is not actually insane.
The other big problem with the 25th Amendment is it requires the VP to agree, and he's not likely to vote his boss out of office.
Just for fun, imagine Elizabeth II dies and Princes Charles and William also have fatal accidents. The next in line is Prince George but he's only 6. He would be King George VII but the Regency Act would immediately kick in because he's not 18 yet, Prince Harry becomes Prince Regent and we would have Almost-Queen Meghan.