The main source of Kimimaro underestimation stems from the fact that he "lost to a bunch of Genin." But when you take a step back and consider that none of those Genin were actually "Genin-level", Kimimaro's performance becomes a lot more impressive.
In Naruto's very first chapter, he beat a Chunin (Mizuki). Sasuke trashed the demon brothers (two Chunin) within the first twenty chapters, and considering Naruto and Sasuke's progression was relatively equal (in Part I, at least) it should become painfully obvious that end of Part I Naruto is very comfortably above Chunin, if not low-Jonin level.
Since the first chapter (where he beat a Chunin), he's gained Rasengan, better Kyuubi control (pertaining to KN0), and has overall improved his physical stats. He is, by definition, stronger than Chunin level. Hell, even Genma at the Chunin exams said that Sasuke was "already at the Chunin level" when he saw him in his fight with Gaara, and this was without 3-tomoe Sharingan and CS1 or CS2 in play.
Rock Lee trashed a superior version of the Sasuke who in turn trashed two Chunin (Sasuke had no Sharingan against the demon brothers whereas he had 2 tomoe against Lee). Drunken Rock Lee was a step ahead of his "base" self, as well. Again, drunk Rock Lee by definition was above Chunin level, at least high Chunin if not low Jonin.
SRA Gaara was outright high Jonin / low-Kage. He had a pseudo-sand source and was using minerals in his sand that made it harder than normal. And considering how fast Konoha tried to replace their Kage, I wouldn't be surprised if Gaara was made Kazekage soonafter that incident (it would be odd if Sand Village was okay with being Kage-less when Konoha was running through Kages like tic-tacs; Tsunade to Danzo to Kakashi back to Tsunade). The mere suggestion that SRA Gaara was Genin level is just lolworthy.
So when you consider that Kimimaro basically survived a gauntlet of what was at least three Jonin level fighters (Gaara was probably low-Kage level), it suddenly becomes a lot more impressive. He outright would have won the battle had his illness waited two more seconds. And when you consider that he essentially just walked out of his deathbed where he was attached to lifesupport with around a dozen tubes, his feat becomes a lot more impressive.
People calling Kimimaro Genin / Chunin level are being intentionally dense. The current ninja war was being dictated by two Genin (Naruto / Sasuke / Kabuto) and a Chunin (Obito). That's how little rank matters when we are dealing with main characters.