Beck, Friday, April 15, at Ground Zero, has presented a quandary to music-scene observers. They wonder: Is he the new Bob Dylan or the Vanilla Ice of slack? The verdict's not in yet, but it seems clear from the early reactions to Mellow Gold, the anti-album just released on the Geffen label, that this singer-songwriter will not inspire indifference. Because the live notices Beck has earned since the song "Loser" became an MTV smash of epic proportions have ranged from terrible to really terrible, checking out this phenomenon in concert may be something of a risk. Then again, who can resist the opportunity to be the first on his block to know from firsthand experience if Beck bites?