why does alternating color of table faintly color vertical lines too?

I think that what you see is mostly aliasing and/or effect of the viewer antialiasing algorithms. Your code, using okular, 1600% magnification, color picked:

If the effect annoys you, you can remove all the vertical lines and play with spaces and styles:

documentclass[11pt]{article}
usepackage[table]{xcolor}
definecolor{lightgray}{gray}{0.9}
usepackage{booktabs}
begin{document}
begin{table}[ht]
caption{default}
begin{center}
rowcolors{1}{}{lightgray}
begin{tabular}{>{bfseries}rrrrrr}
  toprule
  qquad & 1 & 2 & 3 & 4 & 5 
  midrule
  1 & 2.36 & 1.08 & -0.49 & -0.82 & -0.65 
  2 & -0.68 & -1.13 & -0.42 & -0.72 & 1.51 
  3 & -1.00 & 0.02 & -0.54 & 0.31 & 1.28 
  4 & -0.99 & -0.54 & 0.97 & -1.12 & 0.59 
  5 & -2.35 & -0.29 & -0.53 & 0.30 & -0.30 
  6 & -0.10 & 0.06 & -0.85 & 0.10 & -0.60 
  7 & 1.28 & -0.46 & 1.33 & -0.66 & -1.80 
  8 & 0.80 & 0.46 & 1.37 & 1.73 & 1.93 
  9 & -0.75 & 0.28 & 0.51 & 0.19 & 0.58 
  10 & -1.64 & -0.12 & -1.17 & -0.10 & -0.04 
   bottomrule
end{tabular}
end{center}
end{table}
end{document}

The package nicematrix has tools specifically designed to solve that kind of problems.

The environment {NiceTabular} draws the colored cells, rows and columns before the rules. The resulting PDF is much easier to interpret by the PDF viewers and you won't have the effect you see by using the package colortbl (which is loaded by the key table of xcolor).

documentclass[11pt]{article}
usepackage{xcolor}
definecolor{lightgray}{gray}{0.9}
usepackage{nicematrix}


begin{document}
begin{table}[ht]
caption{default}

begin{center}

begin{NiceTabular}{r|rrrrr}
CodeBefore
   rowcolors{1}{}{lightgray}
Body
  hline
 & 1 & 2 & 3 & 4 & 5 
  hline
1 & 2.36 & 1.08 & -0.49 & -0.82 & -0.65 
  2 & -0.68 & -1.13 & -0.42 & -0.72 & 1.51 
  3 & -1.00 & 0.02 & -0.54 & 0.31 & 1.28 
  4 & -0.99 & -0.54 & 0.97 & -1.12 & 0.59 
  5 & -2.35 & -0.29 & -0.53 & 0.30 & -0.30 
  6 & -0.10 & 0.06 & -0.85 & 0.10 & -0.60 
  7 & 1.28 & -0.46 & 1.33 & -0.66 & -1.80 
  8 & 0.80 & 0.46 & 1.37 & 1.73 & 1.93 
  9 & -0.75 & 0.28 & 0.51 & 0.19 & 0.58 
  10 & -1.64 & -0.12 & -1.17 & -0.10 & -0.04 
   hline
end{NiceTabular}
end{center}

end{table}
end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes).