Eventually, these two options worked. Firstly changing \setupbackgrounds

% BEGIN MP LOGO SETUP

\startuseMPgraphic{mpglogo}
path p;
p := (0,0)--(100,0)--(100,100)--(0,100)--cycle;

path q;
q := p cornered 30pt;
draw q withcolor black withpen pencircle scaled 7pt;
\stopuseMPgraphic

\definelayer
[logo]
[repeat=no]

\setlayer
[logo]
[hoffset=160mm,voffset=17mm]
{\scale[width=25mm,height=25mm]{\useMPgraphic{mpglogo}}}

\setupbackgrounds[paper][background=logo]

% END MP LOGO SETUP, BEGIN LETTER SETUP

\usemodule
[letter]
[style=dinb]

\setupletterlayer
[reference]
[list={name,phone,email,date}]

\setupletterlayer
[address]
[y=55mm]

\setupletterdescription
[enclosure]
[location=top,
leftmargin=0.5em,
symbol=#1,]

\usesymbols
[mvs]

\definesymbol
[1]
[{\symbol[martinvogel 2][PointingHand]}]

\setupletter
[toname={John Doe},
   toaddress={201, Main Street\\23124 London\\United Kingdom},
   fromname={Jansen NV},
   fromaddress={Waalhaven 2200\\2842 KL Rotterdam},
   name={Piet jansen},
   phone={+31(010)4234 4232},
   email={peter.jansen@gmail.com},
   date={\currentdate},
   signature={Pete},]

% END LETTER SETUP, START LETTER

\startletter
[subject={Outstanding Invoices},
   opening={Dear Sir,},
   closing={Kind Regards,},
   enclosure={list:Commercial Invoice,Accounts Statement},]

\dorecurse{3}{\input knuth \par}

\stopletter

Alternatively, adjusting \setupletterlayer

\usemodule
[letter]
[style=dinb]

% BEGIN MP LOGO SETUP

\startuseMPgraphic{mpglogo}
path p;
p := (0,0)--(100,0)--(100,100)--(0,100)--cycle;

path q;
q := p cornered 30pt;
draw q withcolor black withpen pencircle scaled 7pt;
\stopuseMPgraphic

\startsetups[letter:layer:lettermain]
\scale
[width=25mm,height=25mm]
{\useMPgraphic{mpglogo}}
\stopsetups

\setupletterlayer
[lettermain]
[alternative=setups,
x=160mm,
y=17mm]

% END MP LOGO SETUP, BEGIN LETTER SETUP

\setupletterlayer
[reference]
[list={name,phone,email,date}]

\setupletterlayer
[address]
[y=55mm]

\setupletterdescription
[enclosure]
[location=top,
leftmargin=0.5em,
symbol=#1,]

\usesymbols
[mvs]

\definesymbol

[1]
[{\symbol[martinvogel 2][PointingHand]}]

% END LETTER SETUP, START LETTER

\startletter
[subject={Outstanding Invoices},
   opening={Dear Sir,},
   closing={Kind Regards,},
   enclosure={list:Commercial Invoice,Accounts Statement},]

\dorecurse
{3}
{\input knuth \par}

\stopletter

---------- Forwarded message ---------
From: Wolfgang Schuster <wolfgang.schuster.lists@gmail.com>
Date: ma 11 mrt. 2019 om 20:55
Subject: Re: [NTG-context] Meptapost Drawing not showing-up in Context Letter Style
To: mailing list for ConTeXt users <ntg-context@ntg.nl>, Context NTG <contextntg@gmail.com>

Context NTG schrieb am 10.03.19 um 16:59:
> I am trying to get a metapost drawing as logo added to right upper
> corner of a DIN B style letterhead, but it does not show up, just the
> letter itself. Is there anything to change to the layers for the MP toe
> reveal itself?

The letter module uses local settings for the page background to place
all the layer for the header, footer etc. and your own background setup
is overloaded by the module.

What you can do in this case is to use the predefined but unused
lettermain layer to place your logo in the background.

\usemodule[letter][style=dinb]

\startuseMPgraphic{mpglogo}
path p;
p := (0,0)--(100,0)--(100,100)--(0,100)--cycle ;
path q;
q := p cornered 30pt;
draw q withcolor black withpen pencircle scaled 7pt ;
\stopuseMPgraphic

\startsetups[letter:layer:lettermain]
\scale[width=25mm,height=25mm]{\useMPgraphic{mpglogo}}
\stopsetups

\setupletterlayer
[lettermain]
[alternative=setups,
x=160mm,
y=17mm]

\startletter
\dorecurse{3}{\input knuth \par}
\stopletter

Another way to place content in the background is to use the “paper” (or
“text”) background which isn’t used by the letter module and your
settings won’t be overloaded.

Wolfgang