Nope. Simply said it's not in Riot's best interest to punish players for playing ranked.
In fact, playing early might make it easier to get a higher MMR, but for a different reason: you can get used to to the season changes as they roll in, players who skip out on ranked for a while will be behind when they start over. Of course you can use normal games to stay in shape, but my experience is that these are taken far less serious by most players, thus they are far less educating (Skill level on normal games also fluctuate far more, making it hard to properly benchmark new strategies, something vital during changes).
Now on the topic of MMR:
Not only diamond MMR got lowered. Everybody's MMR got lowered, including yours. Effective this means your position in the ranking did not change, instead, the value of MMR changed.
For ease, let's say all MMR got halved:
A diamond player started at 2400 MMR, is now 1200 MMR. You started at 1500 MMR, and 750 now. During your placement matches, you will never play against the former diamond player, he's still 450 points ahead of you.
The only real change is that you can gain MMR a lot quicker. Before you would get say, 10 MMR, that's 0.8% of your total MMR. Now you still gain 10 MMR, but that's effective 1.6%. Over time this will result in you growing back to your old 1500 MMR, but this will take some time. Placement matches do give you a bigger MMR swing than normal games. In fact, i believe all games early in the season give a higher MMR reward/loss, this is simply because you played less games. Rating systems always work like that: the more games you play, the more stable your rating become. This is also why the season reset is important: you now required far less games to get into a higher division when your skill improves.
Now, once you are past your placements, and your MMR is starting to recover towards the old 1500, you do start to run into players that did not play for a while, passing by to the top. My experience from last season is that this is not all that problematic. Mostly because there are not all that many of them, also, you have 4/9 change to get one, the other team has 5/9 change to get one. These players are not diamond players playing vs silvers, more likely they are gold players playing vs silver or platinum players playing vs gold. To be honest, the odds of running into a normal smurf in silver is higher. Overall it just means it gets slightly harder to get to your old ranking.
I think that's also the goal of the MMR re-balancing: All players get cramped together into lower divisions, allowing us all go grow again during the season. Over the course of the entire season there is MMR inflation: In my experience it's easier to get to higher divisions at the end of the season than near the start. However we talk here about 3-4 divisions over the entire season. No reason to not play for the first 8 months of the season. Not playing will cost you far more skill and you wont get back up any faster. I guess they decided it's more fun to start the season at gold V and grow back into gold I, than to start in Plat V and be stuck there the first 3 months. If you are Platinum material, you will get there either way.