maybe on offense, probably not on defense. depends on who else you have in your collection.
mk is a decent choice for a go second team, due to his ridiculous base stats making him super tanky. the problem is that he's incredibly slow, only targets one champion at a time, and doesn't have anything like reduce max hp or block revive that ensures he can bring the fight to a close. on offense you can pick your targets and focus on taking down your enemies one champion at a time but on defense, without you controlling him manually, he's far more likely to just ineffectively target your opponents dps over and over and over.