Why hasn't Joe Biden said anything about lowering the deficit? Why hasn't he repealed Trump's tariffs? Does he just not care?

Repealing Trump's tariffs would tend to raise the deficit, not lower it. 

As for doing something to lower the deficit, Biden has been in office for a little over two months.  That entire time has seen the US embroiled in a public health and economic crisis.  The deficit isn't important.  As the old saying goes, you don't worry about the water bill when your house is on fire. We've got plenty of time to worry about the deficit when there isn't an imminent crisis going on

How do you lower the deficit when you just enacted a 3 TRILLION DOLLAR spending bill??

The top income bracket should pay a 90% federal income tax rate.  That would lower the deficit.

Tariffs are the only way to end slave labor. Biden shares the same humanitarian ideals as Trump. If it weren't for the deficit, GDP growth wouldn't be possible. Both Democrats and Republicans opt for positive GDP growth over a balanced budget.