Scope and Contents NYT Exhibition Review: Whatever the aesthetic merit of Gene Beery's Pop-Conceptual paintings, on view in this delightful miniretrospective, they are certainly funny. Since the early 60's, Mr. Beery has been making word paintings that look and read like signs you see in shop windows. What the words offer, however, are not commodities but mischievous, self-referential messages, like John Baldessari's but goofier. ''Invent your own art kit!'' blares the exuberant text of a 1968 spoof of Conceptualism called ''Be an Artist Tomorrow.'' ''No brains, technique, skill, imagination, etc. needed. Clever system makes everyone an old master in seconds.''In ''Essences Only,'' made this year, Mr. Beery used a thin brush to write in black with apparently casual haste on a white canvas, ''My life is now too short to spend much time on formal painting art concerns. I am simply recording essences.'' Just so.It is tempting to go on quoting other paintings, but it is worth noting, too, the paradoxical...