Notice how both examples are at Marco's expense. (From a Star angle it could be argued her intelligence and/or level of ignorance fluctuates for comedy's sake. But that generally hurts others more than her.)
I've seen some folks who get motion sickness explain that it can be on and off like that, ;ie fine when in something familiar like their own car, or routine commune. But keel over when someone else is driving or something like that. It's like a rhythm thing apparently, the less familiar something is, the higher the chances for nausea.(Either that, or he remembers to have some ginger before going to school and/or family car trips.) That said, unless they expand upon it and/or explain it's always going to feel like a shoehorned-in gag to make him appear awkward and/or weak around his peers and/or crush.
I don't think they ever said/showed anything about Jeremy ever beating Marco directly outside of being higher ranked.(Which they said was mostly a result of throwing his wealth around) Even so it would probably be like >>78472127
said with Jeremy being a big cheater. If I recall right the other students rank lower than Jeremy as well with an orange and blue belt respectively. (I'm not familiar with the hierarchy aside from how they differ depending on style/school?) Also have to remember that said black belt ran away the moment Marco snapped during Banagic Incident. If he actually earned his rank he could've fought off the green belt easily.
As for the green belt maybe Marco started late? Or he's just really really unlucky, and always injured himself whenever a big test came, thus missing out on various opportunities to 'rank up'. It could also be reflective of his overly safe nature. I can sort of picture a younger Marco trying to convince himself that he won't get splinters/diseases/hurt too bad from breaking planks.
But yeah, I hope they put consistency over comedy more often in the second season.