Definitely not a bad mother.
I think many of us who didn't struggle with the lockdown in March 20 are struggling now. My friends with younger kids who did full on homeschooling plus playtime plus outdoor walks are struggling now. I don't get bored, way too much to do that I enjoy, but my motivation to do any of them has been so deep in sub-zero territory that I can only describe this as boredom. The mind numbing, soul destroying tediousness of this lockdown is getting to me. And everyone I know.
So, YANBU. Not at all.
I was a SAHM for 14 years and that age was a huge challenge. At 3, humans are at peak energy levels, so if you're not burning that off, you're going to struggle. I have three boys and I learned the hard way that they needed exercise. So no matter the weather, I forced myself to take them outside. For me that was harder than indoor play - I've always preferred to stay in.
Structure was not only useful, but it was the only way I managed to get anything done. After lunch, my kids all napped. If they didn't want to sleep, they had quiet time in their (childproofed) rooms. Many days I needed that quiet time far more than them.
The one thing I would say about playing with your toddler, is that playing well is a skill children need to learn like anything else. The time you invest in playing with them early on you get back later. Massively. The time you let them play by themselves early on you also benefit from. I had lots of friends who did neither, or one but not the other. And in later years, when mine happily played for hours by themselves, their kids just couldn't entertain themselves - they always needed attention. They either didn't learn how to focus on a game, so nothing appealed to them for long enough or they didn't learn how to do so by themselves.
I discovered this by accident - I felt less guilty if I had played with my kids for two sessions a day (sometimes one or three, but before and after lunch was the norm for us). Plus lots of attention at bath and bedtime.
The play sessions were usually 20 to 30 minutes intense play and then I would leave them to it. I told them I was busy and we'd play again after lunch/at bath time etc. By the time you fit in a trip outdoors, two play sessions and bath and bedtime, that's plenty of attention. It also helps break up the day and kids, at least my kids, thrive on schedules.
As for your phone use. That's really hard right now. We are social animals and right now the phone is one of the few ways we can keep in touch. It's not like we can go out and meet up in a lockdown. If you can, I would follow some of the other excellent suggestions to make it harder for yourself to access certain apps, or to schedule a phone free time.
But you are not a terrible mother. Forgive yourself.