It's a double whammy here in KE.

1. The house is overvalued
2. The high mortgage rate on that overvalued house

I would rather say it depends..
Ideally a mortgage helps one own a home(read property) that you need/want but you can't own/afford now (unless you can from savings et al). So the mortgagor enables you to own it but at a cost (read interest). The challenge is factors behind the cost to incure (principal is constant but interest depends on the rate movement which is determined by both market factors as well as the government behaviour (level of borrowing)).
If one does major prepayments then the cost would be significantly reduced as interest payments will be reduced in a stable interests regime.
The lack of many mortgagors stiffles the uptake to an extent, as most do not have a high appetite for mortgage business hence small allocation limit in their portfolio.
Majority pf bank staff as said would tell its not a worst decision.
etc

Mortgage in Kenya currently not as attractive as can be, all basic comparative analysis aside, I'd say it's especially WORST when you can't let the unit for equal or more than the monthly installments!

So how much are you paying now?

Let's put is simply. Any mortgage with a rate higher than than the rental yield is a bad deal. The rental yield should be even higher than then mortgage rate to pay you back for the extra risk you are taking on.