I see nothing wrong with using ground-up cones as mulch. in fact I'm sure that they are often included in commercial bark mulch. However, I would avoid going overboard with them. If you are just trying to keep weeds down, they will be fine, as they decompose very slowly. However if you expect to get some volunteer seedlings from your flowers it is doubtful that these will germinate due to the bark or cone mulch. Cones and shreddded bark mulch tend to make the soil a little more acid over the long run, but if you like the look of it by all means use them up on the beds in lieu of composting them.