FYI: Your summons get the AVG level of your non-summoned toons.
If you get a new toon and the level of that toon is less than the other toons you have unlocked and levelled, it will literally reduce the level of your summons. This can affect things like landing debuffs. In many cases a timely Blind preventing an AoE will add more to your run than a low-level additional character (especially since they'll die to 1 or maybe 2 AoEs and not be around to contribute.
There is, of course, also RNG. Even with everyone levelled, sometimes the Blind lands, sometimes it doesn't. But if your new toon is significantly lower level than your established toons then, yes, including them can lower your score.