I recently did a rewatch of the first few DS9 series with my mom and i remember thinking at the time that Rom's sudden technical genius did make a weird kinda sense. After all the Ferengi value buisness smarts so Rom's intelligence regarding tech would go unoticed/underappreciated by his own people and since he spends the majority of his time working or hanging out with his own people all of whom view him as stupid because of his lack of buisness sense.
Its entirely plausible that the non-ferengi characters would who only usually encounter Rom when he is working as a waiter and are unlikely to ever discuss technological issues with him would not realise that Rom is in fact a technical genius. I myself have a degree and masters in literature but because of the limited number of jobs in my local area am currently stuck working as a cashier at a small store where most customers behave as though i am stupid simply because i am a cashier. Rom is a waiter, a job that doesnt look as though it requires much skill and doesnt give the employee much if any opportunity to display their intelligence in any area other than balancing stuff on a tray, and Rom is often clumsy or tongue tied so its not surprising that people would believe Rom to be stupid based upon his occupation and outward appearance and what Quark says about him.
Furthermore Rom's technical genius is portrayed as being more of an instinct or intuition about how to fix things or make them run more efficiently with often limited resources rather than through an academic/intellectual knowledge of engineering. Rom himself is not good at expressing himself though he gets slightly better when he gets some confidence but even then he mainly just shows up and does his work. Imho i think Rom has the kind of genius that shows itself in what the person does rather than what the person says and until the later seasons he had very little opportunity to show what he could do engineering wise to anybody other than his fellow ferengi who didnt care.