You're mashing together some different things:
- too many toys
Let's look at these independantly:
Sweets: too much sweets is certainly bad for anyone, especially a child. It's bad for their teeth, and the human metabolic response to sugar is like the response to an addictive drug. We didn't give our child sweets until he was old enough to ask for them and knew what he was missing (for us that happend around 2 and a half). Even now (at 4) we keep a close eye on the sweet consumption, but he has a reasonably healthy attitude to sweets.
I know some parents that kept the sweets away from their kids for a longer time, but I have the impression this leads to the children craving and fetishising the sweets as much as being too liberal with them. You gotta teach kids to eat sweets responsibly.
Tablets: The research indicates that tablets (and screen time in general) is bad for developing children. My wife is an educator and keeps up with the literature on this stuff. I'll try to follow up here with concrete references and professional guidelines, but googline "smartphones and baby development" will give a good indication of the current scientific consensus on the subject. You would do well to keep tables or smartphones out of the hands of your children, as well as keep them away from the television or streaming video or whatever for as long as you can.
Toys: I'm less aware of the scientific research on this subject, but the first noble truth of the Buddha is we are inherently unsatisfied. Give people more and they want more. This is especially true of babies, and if you keep giving kids gifts of rewards all the time, they will quickly normalize to this state, and get more demanding. I see this every Christmas: my kids, and the neighbors' too get a lot of special treatment over the christmas and they head down the spectrum towards spoiled bratty behavior. A few weeks later and they have headed back to their normal brattyness or spoiledness levels. I took my kid to Europapark in the days leading to Easter, and between that and Easter, I saw the same thing -- a reduced ability to cope with disappointment and less pleasure in the things that would normally make him happy. Pretty much human nature.
I had to give my mom strict rules about the sugar and handy stuff. As for the toys, my wife and I gently nudge the relatives to take it easy, and when things get too crowded we pass things on to needier children.
It's a hard topic, but I think you would be wise to chat it over with Grandma in a friendly way. If it's your wife's mother, you might be better off having your wife have the chat.