I am using the exact same method, personally I think it works well enough to do what you need it to.
I think some of the other responses are forgetting a few things. If you are using a HOB filter, let alone DIY co2, as I am, I imagine you are trying to do this tank on a budget. Given that, pressurized co2 and a canister filter with an in tank diffuser are probably not realistic options. Additionally, I am not entirely convinced that this method is not nearly as effective as any other. I say that because with a fine enough airstone, you are starting with very small co2 bubbles to begin with. From there, these small bubbles are being pulled in to your HOB, and being vigorously mixed by the HOB impeller. And even beyond that, if you have fine filter pad in your HOB, the bubbles are being caught by this and held in place while water rushes past, providing even more contact time with the water.
I'm not arguing with the replies saying pressurized co2 with a canister filter and ceramic diffuser is more effective, I just think everyone has a different level of "good enough," and if you're on a budget, at some point you need to realize that stressing out and over thinking things is going to end up distracting you from simply enjoying your tank.