The only way I know to do this is to have a script to trap all messages. If it's not in an "unimportant" channel, then it returns without doing anything. But if it is in an "unimportant" channel, then you send an echo with the -n flag in order to prevent any highlighting of the switchbar icon.
It can be complicated to accurately replicate all the message settings except the one you want. Below are 2 different examples of replacing the incoming messages without touching the switchbar, with 1 being more supportive of extra settings such as the server timestamp, and only showing @nick instead of @+nick if the speaker has both op and voice, etc.
It still doesn't have all settings baked into it. For example, it assumes you have enabled the setting to show the @ prefix, and there's probably some others I've not thought of quickly. Note that it will still highlight the switchbar with the event color for other events, so if you want to selectively disable coloring the switchbar for things like Joins, Parts, Quits, Nick changes, etc, you'd need to script a replacement for those events which disables the event color.
Note that /echo doesn't preserve consecutive spaces, so some ASCII ART won't look nice in these channels. All you should need to do is
Edit the list of channels you want this altered behavior in. If it's not on the list, that channel gets the default behavior.
Decide which of these echoes you want to keep, and delete or comment-out the other
This is just quick and dirty code, so your mileage may vary for situations I didn't think to test for
on ^*:TEXT:*:#:{
if (!$istok(#unimportantchannel1 #unimportantchannel2 ##maroon3,$chan,32)) return
echo -ti2mnbfl # $+(<,$nick(#,$nick).pnick,>) $parms
echo $+(-t,$msgstamp,i2mnbfl) # $+(<,$chr(3),$nick(#,$nick).color,$iif($nick(#,$nick).pnick != $nick,$left($v1,1)),$nick,$chr(3),>) $parms
haltdef
}
on ^*:ACTION:*:#:{
if (!$istok(#unimportantchannel1 #unimportantchannel2 ##maroon3,$chan,32)) return
echo -ti2mnbflc action # * $nick(#,$nick).pnick $parms
echo $+(-t,$msgstamp,i2mnbflc) action # * $+($iif($nick(#,$nick).pnick != $nick,$left($v1,1)),$nick) $parms
haltdef
}
