I think Spike’s mum loved him while she was alive, even though she was painfully aware of how totally lame he really was. Of course, she never said anything, because she didn’t want to hurt his feelings.
Then, when she came back as a vampire, she realized that he was still totally lame. A lot of vampires, once they come back, immediatly hunt down their family and kill them, permanent-like. They don’t have any emotional tie to them any more, and they recognize how much horrible it is to be killed by a family member, making that first kill a sort of once-in-an-unlifetime experience. Spike doesn’t do that. He wants to hang out with his mom. From the POV of another fiend, that’s just pathetic, and the vampire that used to be his mom doesn’t hold back on him.