Are all Timelords geniuses?

It is impossible to write an answer that accounts for every fictional Time Lord, even the ones who aren't explicitly known about.

is EVERY Timelord on Gallifrey as intelligent as the Doctor?

Quite the opposite. High placed Time Lords seems to consider the Doctor to be a relative idiot - in their opinion. To their perception, he is naive and simple and comedic and absolutely not as stoic or pragmatic as they pride themselves to be.

Time Lords are very absent from the rest of the universe, even before the Dalek war. They would rarely if ever travel, usually only as guardians of time. Comparatively, the Doctor is brash, travels around with the wonder and excitement of a toddler, and runs headlong into conflict based on idealistic notions of saving everyone.

Most Gallifreyans present for a specific plot generally have to eventually concede that the Doctor saved the day or did a clever thing, but the general tone is that the Doctor is very unorthodox and his methods are considered simplistic, idealistic and reckless.

Keep in mind that the Doctor stole his TARDIS because he didn't have one, because he didn't pass his TARDIS piloting class (he only barely managed to graduate from school, with a 51% on his second try at his exams). (link).

Even River Song, admittedly clever but not trained in any official Time Lord capacity, knows how to operate the TARDS without that grindy noise, and the Doctor didn't figure that out after (IIRC) about 1000 years since he stole it.

What little we know of his personal life as a Time Lord, he was an academic failure, which supports the idea that he is, by Time Lord standards, definitely not a genius.

But your question seems to be judged from a human perspective, where the Doctor is significantly more advanced than any human.

While we cannot conclusively speak for all Time Lords, based on existing conversations it seems that the Doctor is considered to be on the lower end of the spectrum, at least as perceived by those on the allegedly higher end of the spectrum.

As a minor sidenote, keep in mind that not all Gallifreyans are Time Lords. While it is unclear as to whether there are still non-Time-Lord Gallifreyans around, they did at one point exist.

However, even before they "were gifted" time travel, they were already considered "a proud spacefaring civilisation with pretensions to godhood" (link).

So again, your perspective matters. Relative to humans, Gallifreyans were significantly more advanced. But relative to Time Lords, Gallifreyans are lagging behind.