I would not use it in containers but would dispose of it outside in the soil or on your lawn (where clippings belong).

Your grass clippings have turned to compost. They are absolutely safe to use in flower beds, soil, lawns. I know some potting soils do contain compost, I think your tomato will be fine. I would consider it extra fertilizer. It's decomposed enough now so you won't have to worry about seeds anymore either. It will perk up your flower beds really nice. I just did the same thing not too long ago, and haven't seen any volunteer grass in my flower beds at all.

Hope this helped!

Nor would I consider using it in container soils.

Al