The candle will certainly generate CO2, but as mentioned, it's a matter of ensuring that it's available to the plants.
If you have a co2 meter, you can objectively evaluate.
The effectiveness of CO2 enrichment is primarily dependent on your setup as far as air exchange, use of a lung room, temps, etc.
CO2 can work well in a tent but takes a bit of planning to ensure you're not immediately exhausting what you've created.
I wouldn't sleep well knowing there's an open flame in my grow room; even if done in a "sensible" way.
