The Russians had difficulties welding Titanium alloys at this time so employed thin gauge Stainless Steel instead. Mach 2.8 is a more realistic operational speed for the MiG-25 although it would do 3.2 for a short burst. Any more and the compressor tips would sustain damage. It's interesting that conjecture about what the MiG-25 was and was capable of lead to the F-15 which in turn lead to the MiG-29 and Su-27.
And yes, the J-20 doesn't look right. But neither does the F-35 imho.
The main reason they didn't use more titanium was cost-saving. They did use titanium in areas where the heat loading was greatest, like on the leading edges of the wing, and aluminum in cooler areas.
The Sukhoi T-4 was a prototype for a Mach 3 medium range bomber that was largely made from titanium, but as always it was too expensive for the Soviets.
It seems they'd rather sell the titanium to the imperialist-capitalist western pigs so we could make SR-71's from it.
I've always liked the looks of Sukhoi designs, but I must say the new Sukhoi stealth fighter looks absolutely stunning!