No, the Atlas 551 is not powerful enough to send Juno to Jupiter. From this article on NASA's website:
The Juno spacecraft was launched from Kennedy Space Center in Florida
on August 5, 2011. Juno’s launch vehicle was capable of giving the
spacecraft only enough energy to reach the asteroid belt, at which
point the sun’s gravity pulled it back toward the inner solar system.
Mission planners designed the swing by Earth as a gravity assist to
increase the spacecraft’s speed relative to the sun, so that it could
reach Jupiter. (The spacecraft’s speed relative to Earth before and
after the flyby is unchanged.)
The Earth flyby was a necessary gravitational slingshot to give Juno the final push to Jupiter.
Launch: Juno launched aboard a United Launch Alliance Atlas 551 rocket which is the most powerful Atlas V flown to date. Atlas V supplied a launch energy (C3) of 31.1 km²/s² leaving its payload in a heliocentric orbit with an approximately 2-year period.
Deep Space Manoeuvres (DSMs): When coming close to aphelion, Juno performs two large deep space maneuvers. These events are critical mission events of the 2+ dV-EGA mission trajectory because they target Juno's path back to Earth. Theoretically, the DSMs could be performed with a single burn which would actually provide a better net result in terms of required delta-V, but the burn was cut in half since the Leros 1b main engine of the spacecraft was not qualified for 60-minute burns prior to Juno's launch.
Earth Flyby: For the Flyby, more instruments will be turned on to acquire science data from Earth's environment, both, for calibrations and to support other studies. An additional benefit of the flyby is to give mission controllers the opportunity to practice operating the instruments and the spacecraft in general in preparation for the science mission. Earth flyby occurs on October 9, 2013 with the exact timing depending on the final trajectory maneuvers. Juno will be coming as close as 559 kilometers. Perigee occurs 200 kilometers off the southeastern coast of South Africa.
The gravity assist flyby supplies Juno with a delta-V (velocity change) of 7.3 kilometers per second (26,280 kph, 16,330 mph). With its additional velocity, Juno's orbit around the Sun is modified in a way that takes the vehicle's aphelion outside Jupiter's orbit, crossing its orbit at the correct time to intercept the planet. The gravity assist flyby will mark the only time on Juno's mission that the vehicle is in full eclipse as it passes through Earth's shadow. Eclipse time is about 20 minutes and Juno charges its batteries to 100% before the event so that there are no concerns regarding electrical power.