Since Caesar's early politicial career he had always campaigned the "populares" politics. Which is essentially campaigning issues popular with the general public rather than that of the "optimates" which is politics in favour off the senatorial elite. An example of a senator alligning himself with the optimates is Cato which was one of Caesars main political enemies.

Since Caesar was known to be a man pushing through legislations popular with the general public he became in very popular man in Rome. Especially with the plebians. Not so much with many of the patricians.

There are several reasons for Caesar assassination. Not only did the conspirators fear that Caesar was gaining too much power and possibly making himself King, but he was also a major threat to their own political advancement and thus another reason to be assassinated. Brutus and the other conspirators though underestimated just how popular Caesar was with the general public, who saw him as a competent and good ruler. Mark Antony did not make things better for Brutus and his gang when he gave a passionate speech at Caesars funeral which made the public even more angry at the leading people behind the assassination. In the end most of the conspirators ended up fleeing Rome partly out of fear of the Roman mob.

So in summary Caesar was immensly popular with the regular Roman citizen. Much due to his politics, which made him many enemies in the Senate. He remained popular throughout his entire life and after his death.

Source: Caesar by Goldsworthy