That not best, when you bought it it didn't have the cumulative effect of all those short runs but 70k-miles is more than my wife's 2015 and it goes fine, turbo fine, engine not sluggish even when cold (but it is allowed to fully warm up, 90+C oil temperature) before being pushed, 90 hp 1.2 TSI 5-speed. About around 44mpg IIRC, lots of just 2-mile journeys.
What is a short run, does your oil temperature get up to 90+C for a reasonable part of those runs?
Has your car got one of those GDF/PPF ( gasoline/petrol particulate filter or called summat like that) things (I can never remember which ones do)?
As already asked, what are all the tyre pressures like, when were the spark plugs changed, when was the engine air filter last changed, what's the history mileage/time interval of engine oil and filter changes?
Dragging brakes or partially blocked exhaust but you might notice those on oil temperature if not the 90C biased coolant temperature (see a theme here).
Not directly related to this but change or at least check of brake fluid, inspection at least of brakes are more important than anything on the engine - brakes, steering, suspension (all three incudes tyres) then safety electrics, lights, reflective number plates, windows and mirrors (see and be seen) all more important than engine, but back to engine.
Does your start/stop system always activate when it should?
Nobody expects the Spanish Inquisition, but more questions are to get more information, sudden 20% drop in mpg has to have a reason (the actual mpg isn't as relevant in this case as there are so many variables to comparisons) I can easily get my neighbour's car to lower mpg with him as a passenger just for the fun of twinging his natural Yorkshire nature, and also demonstrated to him that (off cruise control which I never use, I did once in the 1980s but I didn't like it) ignoring the cars suggested gear selections (6-manual) can improve mpg as the biological electronic brain knows more than the car's computer program and inputs (well for now anyway).