Can MQ-135 Gas Sensor be used to make DIY CO2 level monitor?
The general answer to your question is no.  Although the sensor responds somewhat to CO2, it responds highly to other things in the air and this will dominate your signal.  This sensor is made for benzene, alcohol, and smoke.  The output of this sensor will not track with CO2 levels in a way that is usable.

Here is a link to a project where somebody tried unsuccessfully to use this sensor for CO2.