your crash is not due of icu version.
look this : (and try to do this command on your system)
obarun@ obarunS6 test [master*] % ldd /usr/bin/sddm
linux-vdso.so.1 (0x00007ffc5c3b7000)
libQt5DBus.so.5 => /usr/lib/libQt5DBus.so.5 (0x00007f71f622c000)
libQt5Qml.so.5 => /usr/lib/libQt5Qml.so.5 (0x00007f71f5c28000)
libQt5Network.so.5 => /usr/lib/libQt5Network.so.5 (0x00007f71f589d000)
libQt5Core.so.5 => /usr/lib/libQt5Core.so.5 (0x00007f71f51c1000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x00007f71f4e39000)
libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f71f4c22000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f71f487e000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f71f4660000)
libdbus-1.so.3 => /usr/lib/libdbus-1.so.3 (0x00007f71f440f000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f71f40fc000)
libssl.so.1.0.0 => /usr/lib/libssl.so.1.0.0 (0x00007f71f3e8e000)
libcrypto.so.1.0.0 => /usr/lib/libcrypto.so.1.0.0 (0x00007f71f3a1d000)
libz.so.1 => /usr/lib/libz.so.1 (0x00007f71f3806000)
libicui18n.so.59 => /usr/lib/libicui18n.so.59 (0x00007f71f3388000)
libicuuc.so.59 => /usr/lib/libicuuc.so.59 (0x00007f71f2fd7000) # # as you can see sddm use the good version
libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f71f2dd3000)
librt.so.1 => /usr/lib/librt.so.1 (0x00007f71f2bcb000)
libpcre16.so.0 => /usr/lib/libpcre16.so.0 (0x00007f71f2962000)
libglib-2.0.so.0 => /usr/lib/libglib-2.0.so.0 (0x00007f71f264e000)
/lib64/ld-linux-x86-64.so.2 (0x00007f71f64b8000)
libicudata.so.59 => /usr/lib/libicudata.so.59 (0x00007f71f0b3b000)
libpcre.so.1 => /usr/lib/libpcre.so.1 (0x00007f71f08c8000)
Icu is not a dependence (or a make dependence) of sddm. So, you can try to recompile sddm if you want but this will change anything about your trouble. Maybe you can take a look at /var/log/sddm.log...