Inflation is not a problem; unexpected changes in the rate of inflation are the problem. There are various reasons, but the most important is that a spike in the inflation rate leads to a spike in interest rates, which is generally harmful to businesses (loans are harder to repay, customers are less able to buy, etc.). There is also a second-order effect: rising interest rates reduce the value of long-dated bonds, which reduces the available investment capital (or worse, it can trigger margin calls and create a "contagion" effect).

Yes that’s exactly my point. People look at absolute value of the stock market but what matters is the relative value to the dollar.