This one is a tricky one. In English, yes, it is definitely uncountable. In Bantu languages, things become interesting because there are languages that have one form just like English ; e.g. Zulu: udaka This was was easy to explain .
The problem now comes in languages which have both singular and plural form. Because I am a native speaker (not of Swahili) , I know the usage is different but had never stopped to think about the differences until now.
Here are examples where both singular and plural exist;
Swahili: tope (sing.) , matope ( pl. )
Ndau : dope (sing. ) , matope (pl. )
Shona : dhaka (sing. ), madhaka (pl.)
I am not sure about Swahili, but in Ndau and Shona, where you are referring to a specific confined mass of mud , for example one in a container or wheelbarrow , the singular form is used. The 'singular' is also used to refer to mortar which is used to hold bricks together. The 'plural' is used for any other mud. I know it wasn't easy to explain or understand but I hope it helped a little.
In rural Kenya, which is where I hear Kiswahili, the singular refers, as you say, to mud confined for a purpose. The plural usually refers to mud in general or mud on the ground or road. That said, I don't think anyone where we are would care what you use. They know you mean mud.
Liquids tend to be in class 6, such as "water" maji, "milk" maziwa etc. They essentially only have a "plural" form, but it's simpler to just think of them as belonging to class 6 whereas many countable words such as dirisha have a singular form in class five and a plural form in class 6. The groupings of singular and plural classes are not quite absolute equivalents of singular and plural.
That being said, yes, apparently there is also the word "tope", but my dictionary lists "matope" as its own headword too, simply saying "(noun 6)".