You don't need to worry!
Apart from periodic maintenance i.e. Plugs, CVT, COOLANT, BATTERY, TYRES, AIR FILTER, BRAKE SHOES, ETC.
The culprit is high temperature, fuel quality and congestions on road/ traffic, idle on ac, frequent small trips that resulting mira bad average.
I have dec, 2015 mira imported 2018, 68000kms driven was giving me under 12kmpl while driving in a locked down colony with speed limits of 25~40km/hr, well may that have been due to being on standby on AC for about 30 mins.
Now, as situation is bit relaxed on few trips to nearby city where i can cruise to 90km/hr, its displaying 14.6kmpl while still use to idle a lot on non-stop AC.
THIS MIRA HAS STRONG ENGINE, JUST STICK TO SAME ENGINE OIL. I'M using TOYOTA IMPORTED jaoan black and red tin of 5w30 SN/CF.