The math fonts for Concrete are in the old-fashioned METAFONT format. Today, these compile to Type 3 bitmap fonts, which look pixelated. This is true even if you use the most up-to-date Concrete package for PDFTeX, ccfonts. (The concmath package has not been updated since last century.)
One solution is to use the same fonts Donald Knuth himself did. The Concrete font was originally designed for his textbook Concrete Mathematics: A Foundation for Computer Science. The math font he originally used with it in that book was Hermann Zapf’s Euler. (Back then, he was using rasterized fonts for a laser printer, but people have since turned them into into outline fonts that look better on a modern screen.)
\documentclass[english]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc} % The default since 2018
\usepackage[exscale]{ccfonts}
\usepackage{eulervm,eufrak,eucal}
\usepackage{babel}
\begin{document}
Hey!$\lambda\beta\gamma\langle\psi\rangle$
\end{document}
If you can convert your source files to UTF-8 and compile in LuaLaTeX or XeLaTeX, you can use the Greek letters from the OpenType CMU Concrete in math mode. Here, I use GFS Neohellenic Math, another slab-serif font, for all other math symbols.
\documentclass[english]{article}
\usepackage{unicode-math}
\usepackage{babel}
\defaultfontfeatures{Scale=MatchLowercase}
\setmainfont{CMU Concrete}[Scale=1.0]
\setmathfont{GFS Neohellenic Math}
\setmathfont{CMU Concrete Italic}[range=it]
\setmathfont{CMU Concrete}[range=up]
\begin{document}
Hey!$\lambda\beta\gamma\langle\psi\rangle$
\end{document}
There is also an OpenType version of Neo Euler available, and someone turned an example I wrote here about how to duplicate the layout of Concrete Mathematics into \usepackage[neoeuler]{fontsetup}. You must have this free font to use it.
Finally, Microtype still apparently sells a commercial Type-1 version of the Concrete math fonts. I haven’t used it, but it’s supposedly an outline font that won’t pixelate and works in PDFTeX. (However, publishers that don’t let you use LuaLaTeX or XeLaTeX usually don’t let you upload commercial fonts either.)
