It was shown that during the viltrumite fight, Mark was getting demolished by Thula until he stopped holding back, letting him fight her on what looked like almost equal grounds. There was even a point where the show made it look like he was about to finish her. I love this show, but I just feel like this is so inconsistent and a cheap way to make Mark stronger.
Literally minutes before this, Mark fought Lucan and Lucan was able to hold his head down with one finger. Mark had to be saved from this position by Omni-man. Assuming Lucan and Thula are similar in strength, a holding back mark = weaker than Lucan’s finger and a non holding back Mark = almost as strong as Thula. That is just a craaazzzy difference in power that cannot be explained by holding back.
On Earth Mark has to hold back to keep from killing all his foes, it’s due to his morality, human upbringing, and the fact that he’s genuinely good. The average Viltrumite has none of these drawbacks. They’re ruthless, always going for the kill. Plus its speculated that Marks half human heritage gives him access to adrenaline that Viltrumites don’t usually have, and thus he can briefly propel himself past his usual limits, like Humans.