I think it's mostly unfair. I'll sketch out the basic situation, but feel free to hit me with follow up questions.

While Lee's army ultimately got the worst of the Battle of the Gettysburg, it was no cakewalk for the Union. Three generals commanding corps had been killed or badly wounded (Reynolds, Sickles, and Hancock, the latter probably being the best general in the Army of the Potomac), and the I, XI, and III corps had been rendered combat ineffective by losses. V, II, and XII Corps had also been badly hit, and VI corps had been split up all over the battlefield as reinforcements.

Lee's army was in a strong position on Seminary Ridge; an immediate counterattack risked snatching defeat from the jaws of victory, especially considering the losses in commanders and men and the disorganization of the higher level formations. I remember a really amusing exchange I had with U.S. Grant biographer Brooks Simpson; O.O. (uh-oh!) Howard was the only general who really pushed for an aggressive counterattack and pursuit after the Battle of Gettysburg, and he had become a good guide for what not to do, given his less-than-glorious previous career. It's also important to remember that Meade had only been appointed commander less than a week before; it was his first experience commanding an army.

Lee managed his retreat quite skillfully, even with the Potomac river at his back swollen by rain. He successful extracted most of the supplies his troops had pillaged from the fertile Cumberland Valley at one ferry crossing, and when he found the pontoon bridge he'd ordered had not been built at Williamsport, he quickly established a strong fortified line of earthworks to receive any Union attack while the bridge was being constructed. By the end, he was hoping Meade would be over-aggressive and attack his prepared defenses, but the Union settled into defenses of their own.

To add on to /u/dandan_noodles excellent response, I'll say that armies in the ACW did not ever pursue effectively after battles. Part of it was that generals were relatively good at managing retreats, but more was the lack of effective cavalry in relation to Continental armies of the time period. The only Civil War armies destroyed in battle during the entire war were Pemberton's force at Vicksburg and Lee's army at Appomattox, and neither of those was actually destroyed in the field.

So yes, I think it's a very unfair to criticize Meade for not doing something that nobody else proved able to do in the war.

You could make an argument that Meade should have attempted to follow up on his victory more quickly, but as said in the other comment, Lee was in an excellent defensive position with most of his army relatively rested on July 4, and Meade's army had suffered heavy casualties itself.