Prior to moving into my house two years ago, part of he agreement was that the owners would replace the water main valve. It's been good up until a couple weeks ago. There's drip coming out of it about every 5 seconds. It looks to me like I could put thread seal at that connection. Does that sound reasonable? Or might the problem be worse?
Fortunately, you have a shut off valve ahead of the meter.
That fitting appears to be compression fitting. If the threads are leaking you may have some luck with the thread seal. However, if the compression part of the fitting is leaking thread seal will not help. You would have to replace the 90° and the fitting.