Very plausible. The $135K (or was it $125K? https://www.lotterypost.com/news/298726/4444634 ) is what he claimed to have spent. The total value of the tickets purchased is likely at least double that when factoring in previous winners that he cashed to buy more tickets. Take that into account, and his numerous big wins is even more plausible. It should also be noted the jackpot odds on some of the games he played were relatively good.
With that said, the way he presented wins in his videos could mislead some to believe he was able to choose winners. When in reality he would scratch off the validation code and check it at a retailer beforehand to determine whether a ticket was a winner and for how much. He acknowledged doing this in his comments, but many viewers didn't read and/or understand what he was doing.
Where it gets sketchy, in regards to his YouTube lottery channel, was his implication that he had a "system" of finding winners. On multiple occasions he would put up videos portraying winning tickets being scratched "Live!" without the validation code being scratched. To some viewers, this seemed amazing. However, if one thinks it through a bit, one realizes it's possible he scratched many tickets on video beforehand until he found a "winner" to scratch "live".
Sadly, some viewers seem to buy into his ability to find winners / having a winning system without fully questioning what was happening. Many were clamoring for his upcoming book, ready to shell out money believing they'd learn a winning system - maybe they would, but most likely not.
Fortunately, many viewers do use critical judgement and will call out questionable activity. Without going into much detail, an operator of another lottery YouTube channel used to solicit funds via PayPal to buy tickets with and then PayPal back (or buy more tickets at option of the submitter) any winnings. Seemed a cool idea, and many of such videos were entertaining. However, some viewers raised the point of what if the channel owner scratched off a big winner - would they send the winnings back to the submitter or substitute the big winner with a lesser winning / losing ticket...
Needless to say, the Youtube channel owner was eventually banned from accepting PayPal and ceased doing such videos. Unfortunate, if he was truly being honest (don't know either way), but on the other hand, with so many out there desperate to eek out a living, the temptation to scam others, especially in the realm of gambling is so high, that many feel it's better to err on the side of caution and assume the worst until conclusively documented otherwise.
Getting back to Mike Ducker, it seems the bigger question, based on various comments and links posted in this thread, isn't whether the instant wins were legit, but how he acquired the funds to purchase so many tickets.