For defense, definitely the stronger the mummy, the better it is. The player or his troops has/have to and will kill each mummy, obviously, at some point of time. Thus, a stronger mummy costs them more time and effort, does more damage and due to this is more useful. An attacking player can’t afford letting a defending mummy get past his king and main army, or his tent might get killed.
For attack, weaker mummies might make a little bit more sense at least in theory. In practice, however, mummies get killed relatively quickly by high level defenses, unless you either use shield all the time on your mummies, or during some amount of time there’s just no hostile troops nearby. For the first reason, the solution is simply to time your shields well and maybe retreat a bit with hero if necessary to leave some mummies unshielded. For the second reason, well… if there’s no incoming damage for 10 seconds, your mummies won’t die no matter what their level is.
Also, max lvl mummies are not great but still better than the king against e.g. boosted barricades, so they even help you before they die. And of course, you can still use them as meat shields, thus more health = more time to recover for your king.