My dog has a persistent sore lump. Could it be a serious tumor?

I recommend to take Greta to see her veterinarian. While most skin masses are benign, masses that a red/pink, hairless and raised are a concern for a malignant skin tumor called a Mast Cell tumor. It also appears as though this mass is ulcerated regardless and it needs to be removed. Once masses grow to the point of ulceration, they will heal and break open over and over again which can make them prone to infections not to mention oozing/bleeding.
Just like with a human, the doctor will likely take a sample of the mass or remove the mass entirely and microscopically exam it. Thats the only way to know what it is accurately.
In the mean time, protect the mass from trauma and wash it daily with an antibacterial hand soap (rinse it well). You can apply a small amount of coconut oil and rub it well into the skin. This may help the wound to heal a little faster. I hope this helps to point you in the right direction. Please feel free to post any further questions!