Yes, MIDDLER is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, D 2, D 2, L 1, E 1, R 1.

Is it a valid word?

MIDDLER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

being neither at the beginning nor at the end in a series
equally distant from the extremes
of a stage in the development of a language or literature between earlier and later stages
between an earlier and a later period of time

Also: in-between.