There are plenty of computer-generated animated pictures around nowadays, and an increasing number of them are in 3-D format. That means, frankly, that adequacy is no longer enough. And unhappily “Monsters Vs. Aliens” isn’t much more than adequate. Not terrible, mind you—just okay. It has its share of humorous moments and good lines, and especially for nostalgia-hungry boomers familiar with chintzy sci-fi flicks of the fifties, plenty of amusing inside jokes. But though kids won’t hate it, one doubts they’ll fall in love with it either.

A caveat: this reviewer saw the picture in its “conventional” 2D form, and the 3D effects might be so strong that they make it seem appreciably better. But on the basis of content alone, it’s just middle-grade.

The premise is that “monsters” reminiscent of those from B-movies of the 1950s have been kept imprisoned by the US government (represented by General W.R. Monger, voiced by Kiefer Sutherland) in a secret facility. There’s BOB, a gelatinous mass modeled after the Blob (Seth Rogen); Dr. Cockroach (Hugh Laurie), a stand-in for The Fly; The Missing Link (Will Arnett), a lizard that’s a voluble Creature from the Black Lagoon; and the huge, mute Insectosaurus, sort of like a baby-faced version of Mothra. To their number is suddenly added a 50-foot woman—Susan (Reese Witherspoon), a would-be bride grown to enormous size when she’s struck by a meteor just before her wedding to preening TV weatherman Derek Dietl (Paul Rudd). She’s rechristened Ginormica by the general.

Unfortunately, the meteor had its effect on Susan by infusing her with an all-powerful element being sought by the power-mad squid called Galaxhar (Rainn Wilson), who sends his super-sized robot to retrieve it—and trash the earth in the process. Its arrival leads the doofus authorities, led by a dumbbell president (Stephen Colbert), to call out the monsters in a last-ditch defensive move. After a long battle, they defeat the robot at the Golden Gate Bridge, but their hard-won victory, while making them known to the wider world, merely leads Galaxhar to plan an invasion himself, along with an endless supply of clones of himself. That threat takes our heroes to his ship, where the outcome is hardly in doubt though it takes a lot of action and near-misses to get there.

This scenario is a skeleton that a quintet of screenwriters hang a plethora of sight gags, jokes and allusions to old movies on, but most of them are surprisingly flat. You’d think, for instance, that having Colbert on hand would be a guarantee of hilarity, but in fact all the stuff involving the president is third-rate; and Rudd’s turn as Susan’s intended is a drab play on male arrogance, designed to complement yet another pro forma thread—the story of female empowerment his would-be bride represents. In fact, Susan herself is a stale character—the person you’re supposed to root for, but are more likely to find a tepid focal point. Sutherland’s general is just your typical loud-mouthed gung-ho type (think an animated R.Lee Ermey).

That leaves most of the work to be done by the other “monsters,” and among them Insectosaurus is a drag and Arnett’s Missing Link one of those motor-mouthed wiseguys who quickly grow annoying. Laurie’s Cockroach is better, but only Rogen’s BOB hits the ball out of the park (though, one must admit, the pitch dealt to him is an easy one—he’s easily the most likable figure around, and stupidity—his main trait—sells). Part of the problem is one that the picture shares, oddly enough, with “Watchmen”—the monster characters are pale imitations of the originals, rather than the authentic articles. One can only imagine how much more fun it would have been had the picture begun with clips from the fifties movies that might then have “morphed” into their animated counterparts. But maybe rights difficulties made that impossible.

As for the bad-guys, Wilson certainly gives Galaxhar his all, and some imaginative sound effects—the popping of his tentacles as he scampers across the floor, for example—help. But one can’t help feeling that even the squid is a generic character, the sort of villain you might find on Saturday morning cartoons.

Of course, technically “Monsters Vs. Aliens” is way beyond television animation. Many of the action scenes are impressively laid out and rendered, and even the more intimate ones look fine. But at least in the non-3D format, none have the visual elegance that one finds, for example, in “Wall•E.”

The standards for this genre are much higher than they used to be, and unhappily this movie, though amiable enough, just doesn’t scale the heights.