https://www.urbandictionary.com/define.php?term=Manlet

  A man who's height is above midget level, but is well below that of any other man. They tend to be very muscular and wide, without a neck.

is a diminutive suffix, meaning something smaller than a normal thing. e.g. piglet (a small pig), or ringlet (a small ring), or cigarette (a small cigar).

ergo, a manlet is a short / small man.

in context, the parent post is implying that the guys face isn't bad looking and that he probably wouldn't have a problem finding a date as long as he's not short.

common term in the reddit / 4chan / tiktok crowd