Just from reading your question, I would say no. If you added high concentrations of sugar to the growth medium and saw little to no spouting, added lower concentrations and saw some sprouting and added no sugar and saw the most sprouting, this implies that sugar inhibits seed sprouting. Period.

Apparently, some of the seeds were not as affected by the sugar at low concentrations and sprouted anyway (perhaps the sugar was not dispersed evenly on the plate, or there were some bacteria consuming the sugar in certain areas, or certain seeds were more tolerant to the sugar). If low concentrations of sugar enhanced seed sprouting, then you would expect to see more seeds sprouting on the plate with low sugar concentrations than on the plate with no sugar (the control).

Many environmental controls on biological processes do not operate like an on/off switch, but have an effect related to their concentration. This seems to be what his happening in your experiment.

Seeds germinate through a process called imbibition, which is the uptake of water by the seed from the environment. High concentrations of sugar will lower the water potential of the growth medium, reducing the likelihood that water will flow into the seed. You could test this by replacing the sugar in your growth medium with salt or some other substance at identical osmolarities (solute concentrations).

Many things are known to break seed dormancy and promote seed germination. Although these things vary by species and habitat, as far as I can tell, sugar is not one of them. Think about it: what would be the advantage of this? In other words, when would sugar be released into the soil to serve as a signal to the seed that now is a good time to germinate? Things that are known to stimulate germination include: cold, scarification (fire, acid from a digestive tract), light, water, "after-ripening" (an increase in nutrients inside the seed due to fungal infection), chemicals such as nitrous oxide or hormones such as gibberellins. These are all things you could test to see if they enhance germination on lettuce seeds. You might also test to see if they enhance germination in seeds exposed to sugars. (In other words, do they reverse the inhibitory effect of sugar?)

It appears that sugar has a separate inhibitory effect on seed germination, independent of osmolarity. This is probably different for different seeds, but has been studied in Arabidopsis thaliana, the model organism used for plants in genetic studies. This whole area is still being actively studied.