I immediately knew exactly which page and panel you were asking about! (I thought the same as you at first, before breaking it down.)
In this scene, Chibiusa enters the arcade asking, “Is Mamo-chan here?” leading to Makoto’s line:
「委員長のクセに『まもちゃん』『まもちゃん』ばっか言ってっとクラスの皆にバカにされっぞ、ちびうさ」
クセに essentially means “even though”, and is used to tease or criticize. For example, “Even though you’re an adult, all you do is sit around playing video games.”
Since Chibiusa has become the class president (委員長, mentioned earlier in the story), Makoto begins with “Even though you’re the class president”. Because what comes next is くせに, you know it’ll be a criticism. (Note: a noun has to have の before adding くせに.)
The criticism begins as:
「… 『まもちゃん』『まもちゃん』ばっか言ってっと …」
The word ばっか (casual for ばかり) has a meaning like the English “only” as in, “If ‘Mamo-chan, Mamo-chan’ is only thing you say …”
Although, I’d probably translate this part as, “If all you say is ‘Mamo-chan, Mamo-chan’ …”
「クラスの皆にバカにされっぞ、ちびうさ」
「バカにする」 as an expression has the meaning of “to make fun of”. Here, as させる, it’s in what I call the receptive form (as per Cure Dolly’s YouTube videos on it), but most grammar guides call it the passive form (although Japanese doesn’t have a passive form like English does).
「バカにすれる」 would thus be “receive being made fun of”, or as an English passive voice, “will be made fun of”.
In this receptive form, the subject is the one receiving an action (the subject being Class President Chibiusa). The one performing the action that is being received is marked by に, which is 「クラスの皆」 (everyone in class). So, Chibiusa will receive being made fun of by her whole class.
Put together, Mako-chan’s saying, “If you keep saying ‘Mamo-chan, Mamo-chan’ even though you’re class president, you’ll be made fun of by the whole class.”
That’s why in the next panel, Chibiusa has the side-comment handwritten in, 「まこちゃんのいじわるー」
