Yes, Taiwan has mountains but so does Ukraine. But Taiwan's mountains are in the Eastern 2/3 of the country and has very few people but the Western part which is closest to China is relatively flat and inhabits 95% of the population.
Any possible invasion by China wouldn't be fought much in the mountains anyway, much like the Ukraine conflict.......China's desire for Taiwan is based on more territory and is militarily strategic.
As I said earlier, Xi is watching Ukraine intently and if Russia had been able to over-run Ukraine in a few days he might consider invading Taiwan. But since Russia is failing badly in Ukraine, well, Xi probably is having second thoughts.