....yes, falling asleep too fast CAN be a bad thing.
First, you need to understand how it it that we become sleepy. The feeling of sleepiness comes from the accumulation of chemicals within our brains called adenosine. Within the process of energy being used and metabolism that occurs in wakefulness, adenosine levels slowly increase. Therefore, the longer we're awake, the more tired we get. The process of sleep clears this chemical from our brain through the glymphatic system. As a result, when we wake up in the morning the levels of adenosine, and sleepiness, are the lowest and we feel refreshed.
As you are awake, your adenosine levels continue to rise, creating a phenomenon called homeostatic sleep drive. This is also sometimes referred to as sleep load or sleep debt. For example, if you are kept awake for 30 straight hours, at the end of that time you will be extremely sleepy, fall asleep easily, sleep deeply, and you may even sleep longer than you normally would. The levels of adenosine would get quite high and compel you towards sleep. In general, the average person without excessive sleepiness should fall asleep in five to 15 minutes.
If it takes longer than 20 to 30 minutes to fall asleep, this may be a sign of insomnia. If sleep onset, or sleepiness, occurs in less than 5 minutes, this may be an indication of a pathological level of sleepiness. Such a person is falling asleep too fast! It may be an indication of not enough sleep or that a sleep disorder may be undermining sleep quality.