I would agree with you that it means that buses are not exactly on time in the morning. I infer this from the context, because the speaker is stating that they need to be at the airport at a specific time, and wondering whether to take a taxi instead. The unspoken conclusion is that a taxi will allow them to arrive at the right time, while a bus might not.
The word, 'unreliable' would be a more precise substitute for 'not good', as others have suggested. For this speaker at this moment, buses must be reliable in order to be good. So the phrase is imprecise, but acceptable and easily understood in context by native speakers.