"Mno" does seem to have several meanings, but this is a dictionary entry for "too", which suggests "mno" is the right translation:
too (adv) = mno,
that is too small = (hiyo) ni ndogo mno.
all too soon/quickly = epesi/haraka mno.
And if you look up "mno", you get:
mno = much, too much, in excess, excessively, beyond measure:
Asali ni tamu mno = the honey is too sweet.
(It doesn't mention "a lot" or "tasty/delicious" at all, so maybe it is only used informally with that meaning in Tanzania?)
That's still no answer to me. I have not found any way to actually say "this is too sweet", since it (whether you use sana or mno) also means "this is absolutely delicious!". Unless there is a true and clear word for "too" that I have not yet learned (which is quite possible), I would go for other formulations, like "I don't like (too) much ..." or "Please use less ..."