Did you uninstall the extension installed through the extension portal? It might be clashing.
Another issue might be that when you install extension through Nix, GNOME Shell might not notice until you restart it. (press Alt-F2 and enter r on XOrg, log out on Wayland).
Though, in this case, looking in the systemd journal, it appears that the extension is actually broken due to changes in GNOME Shell:
JS ERROR: Extension system-monitor@paradoxxx.zero.gmail.com: Error: Tried to construct an object without a GType; are you using GObject.registerClass() when inheriting from a GObject type?
SystemMonitor_TipItem@/home/jtojnar/.nix-profile/share/gnome-shell/extensions/system-monitor@paradoxxx.zero.gmail.com/extension.js:643:9
tip_format@/home/jtojnar/.nix-profile/share/gnome-shell/extensions/system-monitor@paradoxxx.zero.gmail.com/extension.js:901:27
SystemMonitor_Cpu@/home/jtojnar/.nix-profile/share/gnome-shell/extensions/system-monitor@paradoxxx.zero.gmail.com/extension.js:1199:14
createCpus@/home/jtojnar/.nix-profile/share/gnome-shell/extensions/system-monitor@paradoxxx.zero.gmail.com/extension.js:1366:20
enable@/home/jtojnar/.nix-profile/share/gnome-shell/extensions/system-monitor@paradoxxx.zero.gmail.com/extension.js:2319:26
_callExtensionEnable@resource:///org/gnome/shell/ui/extensionSystem.js:166:32
loadExtension@resource:///org/gnome/shell/ui/extensionSystem.js:348:26
_loadExtensions/<@resource:///org/gnome/shell/ui/extensionSystem.js:590:18
collectFromDatadirs@resource:///org/gnome/shell/misc/fileUtils.js:27:17
_loadExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:569:19
_enableAllExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:599:18
_sessionUpdated@resource:///org/gnome/shell/ui/extensionSystem.js:630:18
init@resource:///org/gnome/shell/ui/extensionSystem.js:55:14
_initializeUI@resource:///org/gnome/shell/ui/main.js:254:22
start@resource:///org/gnome/shell/ui/main.js:143:5
@<main>:1:47