Walter Matthau's flinty heart is one of the film industry's natural resources, and should not be carelessly squandered. But the quality of what has been allowed to be filmed worming its way into that heart has been shocking lately.

Jack Lemmon as a teary Jello-bowl of self- pity, tugging away at a murderous Matthau in "Buddy Buddy," was bad enough. Now, in "I Ought to Be in Pictures," we have a young actress named Dinah Manoff portraying, only too convincingly, a pathetically brash and charmless girl pursing the Matthau character because she is his abandoned daughter.

This is a Neil Simon script, adapted from his play. But in place of the staccato barrage of wisecracks one expects from Simon, there is a muddy stream of bathos. Even before the credits are shown on the screen, the tone is set with the streetwise heroine plunking herself down in a graveyard and demanding, "Grandma, did I wake you?" Libby, it seems, keeps up a running patter with her grandmother, who has been dead for six years, and the question of whether she understands that the grandmother does not literally reply to her is one of the running jokes of the film.

Others include her failure to lose her virginity, and the embarrassment of her cynical old father when she asks him such questions as what it was like making love to her mother.

But more pathetic even than the jokes is the serious theme, of whether the tough girl can win over her tough father. This runs to the lowest level of psycho-fashion, with such questions as "What is it about attachments that scare you so much?"

One cannot blame the actors for this. Considering the thinness of the character, Matthau has done the best he could by it, and Manoff, who received a Tony award for playing the daughter on stage, has fulfilled the role's demand of portraying a girl with nothing but perseverance going for her. There is also a sappy character, the father's soothing, selfless mistress, for whom Ann-Margret should not be blamed.

I OUGHT TO BE IN PICTURES -- At the AMC Carrollton, Crofton Cinema, Jenifer Cinema, NTI Tysons Corner, Loehmann's Plaza, Roth's Seven Locks, Showcase Fair City, Springfield Mall, Village Mall and Wheaton Plaza.