My mortgage rate is many times higher than the interest rate on any of my savings accounts. I can try to get better accounts like an Money Market or a Certificate but I'd have to drop like 100k, which I don't have, just to get a rate comparable to my mortgage.
My mortgage is young so its currently over twice higher than my other assets. $1k in a certificate gets me $50 in a year. $1k lump to my mortgage saves me $2k in future payments. $1k extra annually saves me $15k. If I kept reinvesting that certificate I'd earn about $4K over the same timespan.
As I see it, I should be dumping much money into my mortgage as I can afford right now. Maybe when the account gets smaller than my other assets that'll change. But for now the only thing stopping me is having some actual liquid savings.
If the interest on the mortgage is higher than the return you'd expect on anything (including reinvesting dividends etc) then yeah, put that cash into the mortgage. Maybe there's a market crash and fire sale that you'd miss by doing so, etc.
It's everyone for themselves to some extent so do your spreadsheet. It shouldn't be this way but it might save you a lot of time down the road for not a ton of time up front.
Also factor this in:
There is zero guarantee that the interest rate on your mortgage won't go up, unless you're on a fixed rate that you are on track to repay in full in time.
Yes, your investments may bring in some cash and you might do well out of it but if your mortgage interest rate increases that gain could quickly be gobbled up.