There's a few sides to this. Part of the Mosin's reputation is a more recent phenomena that's largely the result of it being imported in huge quantities to the US civilian market after the fall of the Soviet Union.

But let's get to the actual flaws of the Mosin, because there were plenty. The core of the issue was the bolt design, which consisted of several camming surfaces. As the bolt wore down over the life of the weapon, the camming surfaces ended up having more "give" to them and made for a "mushy" bolt. The bolt was less intuitive to take apart than contemporaries, and its safety, while effective, was more difficult to actuate and required a fair degree of dexterity. The rimmed cartridge itself caused issues (as it did in other rifles with rimmed cartridges), although the magazine itself had a pretty ingenious system for eliminating rimlock. The cartridge required a clunky stripper clip design that's fairly notorious for being difficult to use.

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Other issues were less integral to the gun's design and more high-level production decisions. Things like the stock and bayonet design in Russian models left a lot to be desired, and, unlike most other service rifles of its era, the Russians never developed a universal "short" rifle, instead using something only a few inches shorter than a "long" rifle up until almost the end of WW2. The stock's big problem was its lack of a semi-pistol grip. Though it made the gun marginally cheaper to produce, it's a minor quality of life improvement that makes a big impact on shooter control and comfort. The bayonet was a bit more complicated - there's nothing necessarily wrong with a spike bayonet - but it generally boils down to how tightly the bayonet sits on the gun (on most variants) and the decision to zero guns with bayonets attached. The M44 Carbine somewhat solves the bayonet issue by making it a permanent fixture, but in doing so it increases the weight to make it slightly heavier than the M91/30 it was replacing.

But these issues (and a few more) weren't really an integral problem with the design. Finnish Mosins are a good example of the potential of the designs. The Finns refurbished their Mosins into several models, and most tend to be held in higher regard than their Russian counterparts. In general, changes include revised stocks with semi-pistol grips, improved triggers, and blade bayonets (though details vary between variants) .

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As far as crudeness goes, a lot of that comes from the huge demand for rifles during WW1 and WW2. Just prior to WW2, a number of simplifications were made to the Mosin design to streamline production, the most obvious being the transition from hexagonal to round receivers. During WW2, more corners were cut to keep costs low. Most obvious in wartime guns (1943 seems to be the sweet spot in my experience) are brass fittings replacing steel on sling fittings and hanguard caps.

Follow-up: why does it seem that Russian weapon designers have no concern about sight radius? From the Mosin to the SKS to the AK series and even the SVD, it seems like all their infantry firearms have the rear sight posted up in front of the receiver giving them a disadvantage in sight radius and therefore accuracy.