This question really has two parts to it: 1) did votes "cast" by deceased voters give LBJ enough votes to push him past Stevenson; and 2) were the votes of deceased voters what led to LBJ winning the 1948 election.
The answer to the first question is unequivocally yes. Some number of deceased voters cast votes for LBJ which led to LBJ having more votes than Stevenson. Robert Caro even found the election judge for the infamous precinct 13 who admitted as much in an interview for the book "Means of Ascent." There are also sources who point out that Stevenson was procuring illegal votes as well.
The answer to the second question is a little trickier, but is probably yes and no. Caro and Robert Dallek's book "Lone Star Rising" deal with the contested votes, but as Texas lawyer Josiah Daniel argues Caro and Dallek don't do the litigation that occurred after the election justice.
Mr. Daniel went into detail in a law review article on the legal machinations that led to LBJ ultimately prevailing in the ensuing litigation. Mr. Daniel's article does an excellent job of explaining the legal arguments mustered by Stevenson and LBJ, as well as explaining how the litigation went from a state district court, to a federal district court, to a failed attempt at getting a mandamas (a fancy word for a court order) from the Texas Supreme Court, to the Fifth Circuit Court of Appeals, and ultimately Supreme Court Justice Hugo Black.
The crux of the election came down to the motion to stay the proceedings before the federal district court before Justice Black which:
argues in six fairly succinct points that the trial court lacked jurisdiction because the dispute
is, in essence, an “election contest under the laws of the State of Texas,” that Federal Courts have
no jurisdiction to adjudicate the validity of certifications of the authorized officers of the State
Democratic Party, that Johnson held a vested right to be certified as the nominee of the
Democratic Party, that 28 U.S.C. § 1344 deprives Federal Courts of such jurisdiction, that the
cause of action “does not arise under the laws of the United States and does not involve property
or civil rights protected by the laws of the United States but relates only to political rights,” and
that the subject matter lies solely within the jurisdiction of the United States Senate. The Motion
also relies prominently on Sterling v. Ferguson. Finally, the Motion pleads that a stay “will in no
way prejudice plaintiff’s rights if on final determination, it is held that the Federal Courts have
jurisdiction to determine whether . . . Johnson’s name should appear on the official ballot.” Source.
So, yes, deceased voters contributed to LBJ's victory because they put him ahead in the final vote count, however, LBJ's lawyers ability to prevail in the ensuing litigation led to LBJ's election. As Daniel puts it "[t]he lawyering of the two teams did not produce any jurisprudential landmark.
The significance for LBJ, however, was enormous; but for the legal victory, he would not have
become U.S. Senator and then Vice President and President."