Indeed, "it could be completely wrong". Well, not completely, if there is precedent that's hard to argue with, but treating -ulla rubs me the wrong way, not the least because I've read mention--nothing specific yet--that -el in Germanic might be a dual suffix, and I think this is reflected in -sel, English -else, which are rather deemed instrumental; otherwise, cp eg. En. ladle (viz "-el ("agent suffix")"), cp Ger Kelle "ladle". This makes sense insofar duals were also used for corresponding pairs, I think modulus might be an example. Shoe is my primary example of things that come in contrasting pairs (left and right), thus cp caligula.
Quite trivially, if *-ter is handled as evident Indo-Germanic instrumental suffix with various variants like *-trom, *-tlom, *-dhlom, next to *-mn, another agentive noun suffix, then full grade **-tel-, perhaps from *telH- is imaginable.
Yet, nicknames do not have to follow formal rules, if breaking them in creative ways is half the fun. Suppose German [dumme] Trulla were akin to dominatrix. It would be well acceptable. No, I'm not trolling, the etymologies of certain words like trash, Ger Trödel or threshold (possibly reflecting *-thl, *dhlom) are simply speculative, so far, and DWDS or wiktionary don't etymologize Trulla "[ugly] hag".
Addressing your comment
What does it mean when you use the diminutive form of an adjective?
In general, if the dimuntive derived from an agent suffix or similar the adjective would mark a noun as the thing that beares the agency. So clinic for example is derived from a Greek adjective that would just translate as "bed" whenever it's used as an attributive noun, as in bed ridden (though I can't help imagine it's reflected in clean "sanitary"). Cp bird song, little birdie--a birdy song. This seems especially notable if -ly cannot be explained solely from a sense "body, corpse", thus "bodied".