OP: My question: so should we not reflect on the beauty of something? It's not a bad karma, right? So I thought it will just create attachment (that bounds us to samsara), but I didn't think it would lead to a rebirth in the lower realms?
Reflecting on the beauty of something is fine. It does not generate any bad karma. The problem arises when we generate covetousness or craving for this beautiful thing due to our own ignorance. That will generate non-virtuous karma that can lead to an unfortunate rebirth.
Here is an example of the Buddha reflecting on beauty in a completely faultless way:
“This bhikkhu shines with sublime beauty,
Having a mind utterly straight.
Detached is he, free from fetters,
Attained to Nibbāna by nonclinging.
He carries about his final body,
Having conquered Mara and his mount.”
This is often hard to understand for newcomers to Buddhism. They can misunderstand thinking that the Buddha basically said we should no longer love people or that we should no longer appreciate beauty in the world. This isn't the case. In fact, it is the exact opposite.
It is only through giving up covetousness and craving that we can truly love others. Our craving and greed is actually the enemy of being able to see and appreciate the beauty in this world. Craving and greed create bias and reduce our ability to see the world and people for what is actual. Instead, craving and greed and bias act as filters that harm us from being able to truly love and truly see.
The Buddha taught in the four immeasurables that instead we are to train our minds to perfect the ability to love all sentient beings with absolutely no craving. We are taught to train our minds to love without bias and to generate immeasurable joy at the happiness of others.