What the government has announced so far isn't really "trickle down economics". Yes, there are tax cuts specifically at the high end of the income spectrum (and 'only' to abolish the additional income tax rate created relatively recently in 2010 to go back as things had been since 1988, so not exactly revolutionary) but most of them, certainly in term of cost, are for everyone and for companies.
I think the main worry here is not those tax cuts per se, at a 'normal' time markets would have clapped, it's that they mean higher deficit at a time when national debt has already ballooned during Covid, and that they push inflation higher and the Pound lower when the BoE is trying to do the opposite with steep interest rates rises as a consequence.
As per actual IMF statement: "given elevated inflation pressures in many countries, including the UK, we do not recommend large and untargeted fiscal packages at this juncture, as it is important that fiscal policy does not work at cross purposes to monetary policy."