I don't see that at all if you actually look at the intended purpose of the scene, and now you've gone from memory is weak at damage to memory is bad at combat. Do you believe the mom in that amazing spectacle was suppose to be weak? The complete opposite, she was very strong but it was set up to show her killer was just that much stronger so he becomes a goal for the MC to overcome.
Okay, the first part of your first sentence I don't get, so I'll get to the rest. So first, is Syla weak? Absolutely not.
However, that does not mean all monsters using memory powers are strong in a fight, either in actual combat or in dealing damage. Some may be good at one, the other, or even both, but the implication seems to be that most memory monsters would be better at ambush attacks than a straight fight. The general rule of monsters seems to be that if a monster needs a strong body, either to fight directly, or to use a powerful ranged attack, it generally does not have an equally strong mental defense. Conversely, monsters with powerful mental abilities tend to not have equally strong physical bodies. Syla's fight shows this; every time Syla is struck, she gets eviscerated until she forgets the damage. Meanwhile, Indra either shrugs of every attack, or is capable of dodging Syla's greatest attacks. The fact of this fight is that both of them have minimized the weaknesses of their specific power types; Syla with her forgetting power, and Indra with his faith and conviction in his master's goal.
Neither should be considered as the rule of their monster types, however, that doesn't mean that they can't be compared to those who have, or have had, the potential to become exceptional amongst superhumans and monsters.
So, the intended purpose of the scene, which I'm guessing you mean the fight between the two, can have more than just the meaning of making the MC have a goal of getting strong enough to kill Indra. In fact, if you have played the newest update, you would know that the MC can just say something along the lines of, "Well, now I know what happened," which would somewhat undermine that interpretation, though since I haven't chosen that line yet, I don't know that it definitely undermines it. It certainly shows how powerful both of them are, but it also shows that Syla is smarter than she typically acts. First, she knew that Indra would allow her to set up before the fight because he wanted to fight her at her full power. Second, Syla knew that even fighting against Indra with all of her might, she could never kill him, both due to his power's incompatibility with her body, and due to the fact that if she did, then she would forever be the enemy of the Apostles, and that it would be possible, if not likely, that the Fourth would out and out erase her completely, as she implies that the Fourth could do at anytime currently. So instead, she fought with all of her might for a grey zone, weaken Indra's power reserves and and mental strength enough that she could erase those memories of who he is supposed to kill, thereby keeping a tenuous balance between what the Apostles want, and what she wants. The one thing she miscalculated was just how strong Indra's mental strength was, leading to the last purpose of the scene, showing how kind Syla was. She could have chosen to leave out anyone other than herself, but she didn't. In fact, she specifically chose to let him remember her so that she could choose to use those memories to keep him in the dark even after she was dead. I can't remember the exact lines, but it was along the lines of "they" (Most probably the memories of her) won't let you (Indra) hurt "them" (Most probably those Indra is supposed to kill).
So no, Memory is not necessarily bad at combat, nor is it necessarily bad at physical damage all the time. What I'm trying to say though, is that most of the time, Memory is not as good at physical damage as most other powers, and that Syla and her kin are an exception specifically because they can copy other powers, not the rule of Memory monsters.