It’s height, particularly around the shoulder knobs, that’s the problem with old Rebas. I could squeeze XR4 2.2 or RQ 2.2 in there but they’d buzz when you cornered hard and the fork/wheel flexed. Quite worrying! And nowhere near the clearance for mud.
The thing about using skinny mud tyres in the mud, if you ride in the woods then where you get mud you also get wet roots. Skinny mud tyres are rubbish at wet roots. A bigger tyre might not rail the muddy corners like a narrow one but it’ll drift progressively and not punish you so badly when you hit a root at a bad angle. And as its often muddy in the woods but drier in the open, you’d be changing tyres all the time if you had to use a mud tyre in the mud, or suffer the annoyance of a mud tyre’s drag and lack of grip in the dry.