The poor side armor is not really a mistake, but a compromise to control weight since 99A has added top armor, better power pack and taller vehicle body compared with ZTZ99. PLAGF is relatively strict on weight management, partly because of the diversed terrains in China and partly because of the fact that Chinese tank industry lacks the experience to design and manufacturing heavier tracked vehicles. The 99A with 55~58 tons is already the heaviest Chinese tank of all time so far and engineers suffered a lot to make its moving system reliable.@Orthan
Russian tank technology (Armata is probably the best tank in the world right now, shame that Russia can't afford a T-90-scale fleet of them) is best in the world, true, and the Chinese have made major design mistakes with the ZTZ-99A, using Western turret design without understanding it (relatively poor side armor), but the fact of the matter is, India isn't running Armatas, it's running T-90s.
And Chinese tank design isn't that bad.
Also, I'll point this out for the ZTQ-15. There's a claim going on that with modern ERA, the ZTQ-15 is expected to defeat Mango type penetrators. So, ironically, while the ZTQ-15 is pretty toast against an Abrams head-on, it's good enough to survive against an Indian or Vietnamese T-90S long enough for its ATGM to hit the target.
If you recall, the ZTZ-98 family was designed to counter Abrams tanks, even if 1:1 they might not be completely comparable (note the Abrams is currently heavy as all hell). The ZTZ-96 family, on the other hand, was designed to counter Soviet T-72s. The ZTQ-15 can possibly now be considered the successor to the ZTZ-96; it gives up firepower compared to the ZTZ-96, but it has extremely good weight and mobility characteristics and likely comparable armor due to the current generation ERA.
The real defects should be the poor depression angle (despite its lager and higher turret), the ill-protected gun trunnion area (with an unprotected hole forparallel machine gun, like what we have seen on all Chinese MBTs), and its wedge-shape of frontal turret armor module (due to the wedge-shape of armor module, 99A's frontal turret RHA decreases from the middle to the upper side which makes the upper~top side of the frontal turret more vulnerable against APFSDS attacks).
And no, ZTQ15 can in no way defeat Mango. For ZTQ15 it is literally a fanboy‘s dream to expect any effective protection against kinetic projectiles from a ≥100mm gun. Its armor (even with FY4 added on) may be able to counter 40mm cased telescoped ammunition at best and that is all. It is not designed to counter any MBTs (including T90s or even older T72s) head-on.