Starting a sentence with "Mejor"

If I start a sentence with "Mejor" is that considered correct, informal, or just incorrect? A native told me that the first sentence sounds correct. I included others I pulled from

"Mejor te lo digo después."
"Mejor llame a Control de Animales."
"Mejor no saber nada del futuro."
"Mejor hubiera sido no haberte conocido."

To me it sounds informal, and should be "Es mejor" or "Sería mejor" but I basically don't know at all.

submitted by /u/WoodStandard
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