It's probably one of those words (like "unique") which you should try and avoid using with "more" or "most", at least in formal contexts, because some people may take offence. It depends who you ask, and what exactly is meant. So it's impossible to give an authoritative answer, only to point to sources which say one thing or another or use it in different ways.
Part of the issue with salient is that in this context it has 2 slightly different meanings. Oxford defines it as "Most noticeable or important" but also more weakly as "Prominent; conspicuous" In the former sense it's non-gradable but the latter sense is gradable, and if you search texts for "more salient", you'll find there that "salient" is used in the latter sense. Merriam-Webster quotes two examples from print sources, the first using the word in a singular sense "Marconi's salient achievement" presumably to mean most important, and the latter saying "most salient". (It's noteworthy that the weaker sense is apparently the older, rather than being a word whose meaning decayed over time through misuse. Meanings can shift and words can become more extreme as well as milder.)
The origin of the word is originally in heraldry as a leaping animal; it was extended from there to refer to other things that leap, project, or jump out. (It also has the meaning as a noun in military topics as a bulge or projection in a fortification or line.) There's nothing wrong with saying something is "more prominent" or "more projecting", so in a physical usage it's potentially gradable (maybe less so in heraldry where descriptions are more formalized).
Personally I don't see a problem with it, and I can't find any style guide that mentions it. But, as I said, if even one reputable source says it's non-gradable, you should be wary of grading it, lest someone grade you.