For tVOCs and inaccurate, inferred CO2 readings, absolutely. Sensors like the BME680 or SGP30 are ~4mm square.

For real CO2 sensors, not yet. The smallest accurate sensor I've worked with is the SCD30 and it's much too large to fit into a watch body as-is.

Sensirion has come up with the SCD40 though, which is 12x12x7mm and should make a wearable CO2 sensor feasible.

Well, you could. I have it on 3D printed case, but have no need for mobility. Of course you would need battery and with small (1000mAh or so) battery it could run at least few days with one charge.