I think the biggest problem is that she has, and always will, infantilise him. She wanted a permanent happy, smiling five year old. She could never accept that her son was growing up and I think that's how she failed him, in terms of his mental health.
But rather than learn from it she's now infantilising his grave too. Can she hand on heart say that 12 year old Archie would want these flashing lights, candy canes and snowmen? At 12 most kids don't even believe in Santa. While I didn't ever know Archie, I do know who my daughter was at that age. She wanted to look cool, not cute - she would have been utterly mortified at this being her memory.
It's sad enough when you see mums try and baby their kids like this, to the point where it stunts their development (I know a mum who, despite her child being in Year 2, keeps him in onesies and he still uses a buggy and dummy) - but let this poor child rest in peace, with any tributes to him actually representing HIM and not what Hollie wanted him to be.