This is a huge question; American public opinion changed quite often over the period 1914-1917 in reaction to such things as the Atlantic U-Boat campaign, the Zimmerman Telegraph, the Dublin Uprisings, and other events. The American public at this time - like at any time - was hardly a single mass, either, and that definitely had an important effect.

But broadly speaking - yes, the declaration of war on Germany reflected a pro-British outlook from "the man on the street".

Initially, polling during 1915 indicated the American public was strongly in favour of remaining neutral. Unsurprisingly, this was especially true in places like the mid-west (where lots of German-speaking immigrants had settled), and areas heavily Irish-American. Woman's movements and the clergy were also fairly pleased at the declaration of neutrality, although it would be overstating the case to claim there weren't fractures within these communities as well.

If America was neutral, however, the effect was decidedly pro-British. The economy remained on a civilian footing, which should theoretically have served both sides equally well; however the close proximity of Canada, and the blockade of Germany by the Royal Navy, made it far less risky to provide loans and credit to British manufacturers. Companies saw a chance to extend their influence; JP Morgan, for instance, raised cash for the French war effort despite it being specifically banned by the American government. The fact the America primarily spoke English has also been pointed to as a reason American opinion slowly swung towards supporting the British; English-language sources (newspapers, books, movies) coming from Britain could be quickly and easily distributed, with a penetration German-language sources couldn't match.

The Germans didn't particularly help themselves in this regard either. With their surface fleet bottled up in Kiel, the interdiction of enemy shipping had fallen to primitive U-Boats. The attack on the Lusitania became a symbol of the outrage this type of attack generated; while definitely not "the straw that broke the camel's back" in regard to American entry into the war it is sometimes claimed as, the Lusitania was definitely widely reported and strongly condemned.

The final act in the drama was the Zimmerman Telegraph. It is an extraordinay document; the Germans calculated that they had to resume unrestricted U-boat warfare to cut grain shipments from Canada. They also knew this would probably militarily bring the US into the war regardless of how it was framed. They therefore cabled their Ambassador in Mexico to propose a trade - if Mexico joined the Central Powers, and America entered on the side of the Entente, then Mexico should invade the south of the United States. At the end of the war, Mexico could keep New Mexico, Texas, and Arizona. The British intercepted this and gleefully handed it over at the most opportune moment. I don't have to tell you, but the average Joe American was not happy when they found out.

But in summary, it was the culmination of lots of different events which tipped public opinion into supporting war; by 1917 Americans were definitely keen for a scrap as a result.