Yes, KISSER is a Scrabble word!

Letter values in Scrabble US: K 5, I 1, S 1, S 1, E 1, R 1.

Is it a valid word?

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

someone who kisses
the human face (`kisser' and `smiler' and `mug' are informal terms for `face' and `phiz' is British)

Not playable in Scrabble US: OSCULATOR.