It could be a sign of illness, and if your fish is not eating you’ll want to look for other indications such as a bloated belly, ragged scales or deteriorating fins to diagnose the issue.
However, before jumping to conclusions, there are a few more things to consider. If you are accidentally overfeeding your betta, there is a chance he simply isn’t in the mood for food when you present it. He may pick at old food in the gravel when he gets hungry, which you may not notice. He may be eating plenty, but there is simply way too much food.
Overfeeding is a major cause of illness for bettas, and if you are giving him too much food, there is a good chance he will become sick if he isn’t already. Only feed once per day and only as much as he’ll eat in a couple of minutes.
If you’ve gotten off-track with his feeding schedule, you can try vacuuming the debris and old food from the gravel and then give him a day or two of fasting. He may then be more interested in the food you present, and more likely to eat it when it is fresh.
There is also the chance your betta could be stressed. If there is something lacking in his environment, or the tank is too small, or the water is polluted, or any number of other issues, he could be stressed to the point where he is not eating. Look at his overall situation and check his water parameters. If something seems off, fix it.
Finally, realize that bettas are not necessarily big eaters. A few pellets or a small pinch of flakes once per day is all he needs. Don’t expect him to eat more than that, and if he doesn’t eat for a couple of days, it doesn’t always mean something is wrong.